[ticket/12360] Fix duplicated creation of session when logging out the user

In 7d8b578 this line was introduced to fix a bug with style.php.

Later since 1d42d1b a new session is always created after
killing the old one.

This causes a duplicated creation of the session everytime
the user logs itself out.

PHPBB3-12360
[ticket/12358] Correctly check and change the redirect url for ajax requests

PHPBB3-12358
[ticket/12369] Add template var {BODY_CLASS} to HTML body

http://tracker.phpbb.com/browse/PHPBB3-12369

PHPBB3-12369
[ticket/12369] Added ACP vars

PHPBB3-12369
[ticket/12254] Better language switching on registration page

PHPBB3-12254
[ticket/12254] Stop using deprecated request_var in user class

PHPBB3-12254
[ticket/11352] Redirect the user back to view*.php after disapproving

The user should be redirected back to:
- the topic, if he came from viewtopic and the topic still exists
- the forum, if he came from viewtopic, but the topic does not exist anymore

PHPBB3-11352
[ticket/12369] Removed template vars initial state

PHPBB3-12369
[ticket/12336] Add functions_module.php core event

Add functions_module.php core event to allow adjusting parameters
for custom ACP, MCP, UCP modules (location: function list_modules()).

PHPBB3-12336
[ticket/12254] Use new set_lang var to avoid conflict with change_lang

PHPBB3-12254
[ticket/12254] Add set_lang var changes to subsilver2 files

PHPBB3-12254
[ticket/12370] Do not delete topic notifications when the topic is visible

We should only delete the topic notifications, when the topic is not approved
anymore. This happens, when the post was the last approved but is now
unapproved because the user is on queue, or when it has been softdeleted while
editing.

PHPBB3-12370
[ticket/12370] Fix functional notification test and remove unneeded requests

PHPBB3-12370
Merge remote-tracking branch 'github-nickvergessen/ticket/12358' into develop-ascraeus

* github-nickvergessen/ticket/12358:
  [ticket/12358] Correctly check and change the redirect url for ajax requests
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12358] Correctly check and change the redirect url for ajax requests
[ticket/12372] Use jQuery in javascript dE() function

PHPBB3-12372
[ticket/12372] Unify definition of function dE() across all files

PHPBB3-12372
[ticket/12372] Move dE() function core.js

PHPBB3-12372
[ticket/12370] Add unit tests for topic notifications

PHPBB3-12370
[ticket/12372] Rename JS function dE() to phpbb.toggleDisplay()

PHPBB3-12372