[ticket/12900] Fix potential avatar size box overflow

PHPBB3-12900
[ticket/12856] Copy plupload images to prosilver dir

PHPBB3-12856
[ticket/12856] Remove images from assets/plupload/img/

PHPBB3-12856
Merge remote-tracking branch 'PayBas/ticket/12856' into develop-ascraeus

* PayBas/ticket/12856:
  [ticket/12856] Remove images from assets/plupload/img/
  [ticket/12856] Copy plupload images to prosilver dir
Merge pull request #2876 from PayBas/ticket/12900

[ticket/12900] Fix potential avatar size box overflow