[ticket/11554] Move forum_fn.js to footer

Move forum_fn.js to footer, move find_username to forum_fn.js

PHPBB3-11554
Merge remote-tracking branch 'remotes/cyberalien/ticket/11554' into develop

# By Vjacheslav Trushkin
# Via Vjacheslav Trushkin
* remotes/cyberalien/ticket/11554:
  [ticket/11554] Move forum_fn.js to footer