[ticket/11535] Correctly merge avatar_errors array into primary error array

The $avatar_errors array needs to be merged into the primary $error array
before the group settings get applied. This is currently not the case.
Functional tests for this will be provided by PR #1401.

PHPBB3-11535
[ticket/11537] Adjust error message on ucp group manage to fit rest of ucp

The current look highly differed from the rest of the UCP. Additionally,
this caused the error message to be hardly noticeable at all.

PHPBB3-11537
[ticket/11555] Fix page comparison

Strict comparison should not be used to compare string and number

PHPBB3-11555
[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 'cyberalien/ticket/11555' into develop

* cyberalien/ticket/11555:
  [ticket/11555] Fix page comparison
Merge remote-tracking branch 'marc1706/ticket/11537' into develop-olympus

* marc1706/ticket/11537:
  [ticket/11537] Adjust error message on ucp group manage to fit rest of ucp
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/11537] Adjust error message on ucp group manage to fit rest of ucp

Conflicts:
	phpBB/styles/prosilver/template/ucp_groups_manage.html
Merge remote-tracking branch 'marc1706/ticket/11535' into develop

* marc1706/ticket/11535:
  [ticket/11535] Correctly merge avatar_errors array into primary error array
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