[ticket/8700] Remove unused language strings

Most of the strings were obsoleted by the removal of:
* Imagesets
* Themes
* Templates
* Editor
* Exporter

PHPBB3-8700
[ticket/8700] Use L_COLON instead of adding ":" manually

PHPBB3-8700
[ticket/12365] Do not crop image attachments at 350px

PHPBB3-12365
Merge remote-tracking branch 'VSEphpbb/ticket/12365' into develop-ascraeus

* VSEphpbb/ticket/12365:
  [ticket/12365] Do not crop image attachments at 350px
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12365] Do not crop image attachments at 350px
Merge remote-tracking branch 'nickvergessen/ticket/8700' into develop-ascraeus

* nickvergessen/ticket/8700:
  [ticket/8700] Use L_COLON instead of adding ":" manually
  [ticket/8700] Remove unused language strings
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/8700] Use L_COLON instead of adding ":" manually
  [ticket/8700] Remove unused language strings
[ticket/12379] Remove duplicate labels in attachment uploader when responsive.

PHPBB3-12379
Merge remote-tracking branch 'prototech/ticket/12379' into develop-ascraeus

* prototech/ticket/12379:
  [ticket/12379] Remove duplicate labels in attachment uploader when responsive.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12379] Remove duplicate labels in attachment uploader when responsive.