[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
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
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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