updated CHANGELOG for 2.3.23
update CONTRIBUTORS for 2.3.23
updated VERSION for 2.3.23
bumped Symfony version to 2.3.24
CS fixes
Removed unused imports
minor #12837 [2.3] CS Fixes And Removed Unused Imports (GrahamCampbell)

This PR was merged into the 2.3 branch.

Discussion
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[2.3] CS Fixes And Removed Unused Imports

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

##### This pull request fixes some cs issues in symfony 2.3.

As far as I understand, symfony's coding standards dictate that there should be no blank lines between imports and imports should not be prefixed with a slash. There are one or two other tiny fixes in here too.

Also, we should only import things that we actually *use*. :)

Commits
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0e4f0e7 Removed unused imports
0fc3369 CS fixes
[2.3] More cs fixes
minor #12840 [2.3] More cs fixes (GrahamCampbell)

This PR was squashed before being merged into the 2.3 branch (closes #12840).

Discussion
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[2.3] More cs fixes

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

##### This pull request fixes some more cs issues in symfony 2.3.

I should be done with cs fixes to the 2.3 branch now. Sorry I missed these in my last pull.

Commits
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b034b08 [2.3] More cs fixes
use FlattenException from the Debug component