Extensions as 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. Would be highly beneficial if you want to use extensions from external repositories and want to keep them separate from the actual phpBB files.