[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/12375] Fix broken attachment deletion.

The response is already parsed and when running it through parseJSON returned
null prior to 1.9. Now there's an actual error thrown, so the function is
returning and thus leaving the attachment row intact.

PHPBB3-12375
[ticket/12356] Assign vars preventing Plupload from loading in PM editor.

PHPBB3-12356
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/11098] Add mark/unmark all options to remember me keys

PHPBB3-11098
[ticket/11098] HTML Validity fix to Notifications in UCP

PHPBB3-11098
[ticket/12379] Remove duplicate labels in attachment uploader when responsive.

PHPBB3-12379
[ticket/11098] Update subsilver2 to also include mark/unmark all buttons

PHPBB3-11098
[ticket/11098] Fix error reporting on the autologin key pages

PHPBB3-11098
[ticket/12320] No longer allow Travis CI HHVM environment to fail.

PHPBB3-12320
[ticket/12320] Install a nightly build of HHVM.

PHPBB3-12320
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.
Merge remote-tracking branch 'prototech/ticket/12356' into develop-ascraeus

* prototech/ticket/12356:
  [ticket/12356] Assign vars preventing Plupload from loading in PM editor.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12356] Assign vars preventing Plupload from loading in PM editor.
Merge remote-tracking branch 'prototech/ticket/12375' into develop-ascraeus

* prototech/ticket/12375:
  [ticket/12375] Fix broken attachment deletion.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12375] Fix broken attachment deletion.
Merge remote-tracking branch 'vsephpbb/ticket/11098' into develop-ascraeus

* vsephpbb/ticket/11098:
  [ticket/11098] Fix error reporting on the autologin key pages
  [ticket/11098] Update subsilver2 to also include mark/unmark all buttons
  [ticket/11098] HTML Validity fix to Notifications in UCP
  [ticket/11098] Add mark/unmark all options to remember me keys