[2.3] Examples that points to the DataCollectorListener should be removed.
[Security] Added remote_user firewall info and documentation for pre authenticated firewalls
[Reference][Configuration] Removed version added for remote_user
rebased using x509 pr, added remote_user pre authenticated part
fix missing backtick, rephrased bottom note
fixes missing backtick
Added a note about the total deprecation of YUI
Fixed the official name of Node.js
Reverted removal, removed API links instead
tweaks to the contribution chapter
add note about parameters in imports
Added note about ProgressBar changes
add description for the `validation_groups` option
Added feature freeze dates for Symfony versions
minor #4259 Added feature freeze dates for Symfony versions (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added feature freeze dates for Symfony versions

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

As requested by @webmozart [via Twitter](https://twitter.com/webmozart/status/514365470922014720).

Commits
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d157a14 Added feature freeze dates for Symfony versions
swapped comment and opening in xml configuration example
[#4003] A few more form_themes config changes
Fixed typo
minor #4263 Fixed typo (zebba)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4263).

Discussion
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Fixed typo

Commits
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23fdef3 Fixed typo
Fix error in code