[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.

http://tracker.phpbb.com/browse/PHPBB3-11283