[ticket/11283] Extension manager follow symlinks.

All extensions are located in the `phpBB/ext` directory,
however the `phpbb_extension_manager::all_available()`
method only looks into actual directories and ignores symlinks.

Add the `RecursiveDirectoryIterator::FOLLOW_SYMLINKS` flag to
the `new RecursiveDirectoryIterator` call so that you can store
extensions in a different location and use symlinks so that
phpBB can recognise them.

PHPBB3-11283
[ticket/10805] Clear loading alert timeout after ajax request finished

The timeout for the "request timed out" popup should be cleared if it
finished. Since it is currently not cleared, the timeout alert appears as
an extra overlay if another ajaxified function is ran within 5 seconds of
the initial function call. This patch will take care of clearing the
timeout if either the success (function return_handler()) or error
(function error_handler()) functions are called.

PHPBB3-10805
[ticket/10805] Compare phpbbAlertTimer against null

PHPBB3-10805
[ticket/11302] Correctly select first timezone or selected timezone

While registering, we should default to a given timezone. By selecting the
first timezone by default, this is fulfilled. This doesn't happen
currently, as only the "Select a timezone" selection appears.
If a user selects a timezone during the registration process we should
also make sure that the selected element is still selected; even if we
have to return to the registration page, i.e. if there was an error while
submitting the form. This is currently not the case.
With this patch the javascript code will behave correctly.
Additionally, a duplicate inclusion of timezone.js has been removed as it
was not needed.

PHPBB3-11302
Merge PR #1176 branch 'marc1706/ticket/11302' into develop

# By Marc Alexander
# Via Marc Alexander
* marc1706/ticket/11302:
  [ticket/11302] Correctly select first timezone or selected timezone
Merge PR #1153 branch 'marc1706/ticket/10805' into develop

# By Marc Alexander
# Via Marc Alexander
* marc1706/ticket/10805:
  [ticket/10805] Compare phpbbAlertTimer against null
  [ticket/10805] Clear loading alert timeout after ajax request finished
Merge remote-tracking branch 'erikfrerejean/ticket/11283' into develop

* erikfrerejean/ticket/11283:
  [ticket/11283] Extension manager follow symlinks.