[ticket/8700] Remove unused language strings

Most of the strings were obsoleted by the removal of:
* Imagesets
* Themes
* Templates
* Editor
* Exporter

PHPBB3-8700
[ticket/8700] Use L_COLON instead of adding ":" manually

PHPBB3-8700
[ticket/12349] Update license header in several new files

PHPBB3-12349
[ticket/12349] Fix Sniffer for @license tag

PHPBB3-12349
[ticket/12083] Fix Select All in CODE bbcode with only one char

PHPBB3-12083
[ticket/12362] Throw exception in schema generator on unresolvable dependency

Make sure we throw an exception in the schema generator if we come across an
unresolvable dependency. Otherwise we'll get stuck in a infinite loop that
needs to be cancelled by the user or the maximum execution time.

PHPBB3-12362
[ticket/12364] Add template identifier var to all missing pages

PHPBB3-12364
[ticket/12365] Do not crop image attachments at 350px

PHPBB3-12365
[ticket/12362] Add tests for schema generator

PHPBB3-12362
[ticket/12362] Do not use database test case as it's not needed

PHPBB3-12362
[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/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/12369] Removed template vars initial state

PHPBB3-12369
[ticket/12361] Replace the Google logo.

Currently, a link to Google and its logo are displayed in the
language/en/help_bbcode.php file to explain how to correctly format tags
and to add an image to a post. However, the Google logo is outdated and
Google does not allow the use of its logo on third-party websites. Plus,
that overload their servers. I used the blue phpBB logo to replace it
because the grey one with transparent background is hardly visible.

Originally, I just refreshed the old Google logo with the new one
(https://www.google.com/images/srpr/logo5w.png) but Joas proposed to use
the phpBB logo, and I agree with him.

PHPBB3-12361
Merge branch 'ticket/mael/12361' into develop-ascraeus

* ticket/mael/12361:
  [ticket/12361] Replace the Google logo.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12361] Replace the Google logo.
Merge remote-tracking branch 'Elsensee/ticket/12360' into develop-ascraeus

* Elsensee/ticket/12360:
  [ticket/12360] Fix duplicated creation of session when logging out the user
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12360] Fix duplicated creation of session when logging out the user
Merge remote-tracking branch 'paybas/ticket/12364' into develop-ascraeus

* paybas/ticket/12364:
  [ticket/12364] Add template identifier var to all missing pages