[Form] Remove "value" attribute on empty_value option

Today we faced a very strange issue with the newest Blackberry 10 browser, it was not submitting our forms. Finally we found that in a ```select``` element, if you have a disabled option, it can't have a value or the HTML5 validator will crash and not submit the form. Of course, setting the ```novalidate``` option for the whole form also solved the issue.

Although I know this must be an issue with the WebKit version the BB10 has it can easily be solved in symfony with this change. In fact, it does make sense since we already have a disabled option with no value if the ```preferred_choices``` are not empty and a ```separator``` is set
[Validator] Moved constraints Optional and Required to the Constraints\ namespace
merged branch acasademont/patch-1 (PR #7678)

This PR was merged into the master branch.

Discussion
----------

[Form] Remove "value" attribute on empty_value option

Today we faced a very strange issue with the newest Blackberry 10 browser, it was not submitting our forms. Finally we found that in a ```select``` element, if you have a disabled option, it can't have a value or the HTML5 validator will crash and won't submit the form. Of course, setting the ```novalidate``` option for the whole form also solved the issue.

Although I know this must be an issue with the WebKit version the BB10 has, it can easily be solved in symfony with this change. In fact, it does make sense since we already have a disabled option with no value if the ```preferred_choices``` are not empty and a ```separator``` is set

Commits
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9e849eb [Form] Remove "value" attribute on empty_value option
[Form] Improved FormRenderer::renderBlock() to be usable outside of form blocks
[Form] Implemented form processors
[Form] Removed trigger_error() for deprecations as of 3.0
[Form] Moved upgrade nodes to UPGRADE-3.0
[TwigBridge] Removed duplicate entries from the CHANGELOG
[PropertyAccess] Changed PropertyAccessor to continue searching when a non-public method/property are found
merged branch bschussek/issue5493 (PR #6522)

This PR was merged into the master branch.

Discussion
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[2.3] [Form] Implemented form processors

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: partially #5493
Todo: -
License of the code: MIT
Documentation PR: symfony/symfony-docs#2092

Commits
-------

11fee06 [TwigBridge] Removed duplicate entries from the CHANGELOG
68f360c [Form] Moved upgrade nodes to UPGRADE-3.0
01b71a4 [Form] Removed trigger_error() for deprecations as of 3.0
81f8c67 [Form] Implemented form processors
0ea75db [Form] Improved FormRenderer::renderBlock() to be usable outside of form blocks
merged branch bschussek/property-access-public-check (PR #7711)

This PR was merged into the master branch.

Discussion
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[PropertyAccess] Changed PropertyAccessor to continue searching when a non-public method/property are found

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | not necessary

Commits
-------

2a666cb [PropertyAccess] Changed PropertyAccessor to continue searching when a non-public method/property are found
[Form] Renamed option "virtual" to "inherit_data"
[Form] Moved parent data inheritance from data mappers to Form
[Form] Fixed the deprecation notes for the "virtual" option
Moved TypeTestCase to it's own namespace
Moved FormIntegrationTestCase and FormPerformanceTestCase to the Test namespace
Update composer.json

symfony/debug was not mentioned in the replace list
merged branch sstok/patch-6 (PR #7723)

This PR was merged into the master branch.

Discussion
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Update composer.json (symfony/debug was not mentioned in the replace list)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

symfony/debug was not mentioned in the replace list

Commits
-------

57d476b Update composer.json
merged branch WouterJ/form_test_namespace (PR #7659)

This PR was merged into the master branch.

Discussion
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[Form] Moved TypeTestCase to the Test namespace

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #7175
| License       | MIT
| Doc PR        | symfony/symfony-docs#2500

Todo:
 - [x] Make a Doc PR
 - [x] Update the changelog and UPGRADE-3.md files

As discussed in #7175, since this class is documented as a practise to test your form types, it's good to move it to the `Test` namespace.

I am not sure about the deprecation, I thought things deprecated in 2.3 are removed in 3.0. Also, I think we shouldn't trigger a E_USER_DEPRECATED message.
Please correct me if I'm wrong, it's my first PR for the core code.

Commits
-------

8da6129 Moved FormIntegrationTestCase and FormPerformanceTestCase to the Test namespace
e46f841 Moved TypeTestCase to it's own namespace
merged branch bschussek/move-existence-constraints (PR #7701)

This PR was merged into the master branch.

Discussion
----------

[Validator] Moved constraints Optional and Required to the Constraints\ namespace

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#2512

Commits
-------

a868048 [Validator] Moved constraints Optional and Required to the Constraints\ namespace