[ticket/11328] Add language variables for buttons

New language variables for buttons

PHPBB3-11328
[ticket/11328] Replace long buttons text with short text

Replace long buttons text with short text, like it was on images

PHPBB3-11328
[ticket/9492] Retain custom ranks and avatars when setting users default group

PHPBB3-9492
[ticket/9492] Add unit tests for custom ranks and avatars

PHPBB3-9492
[ticket/9492] Fix undefined user_avatar_* values when updating the group avatar

PHPBB3-9492
[ticket/9492] Ensure to update all avatar values when the avatar is changed

PHPBB3-9492
[ticket/9492] Ensure to delete the avatar/rank data when we change it

PHPBB3-9492
[ticket/9492] Fix typo in SQL query

PHPBB3-9492
[ticket/9492] Fix missing phpbb_container in unit tests

PHPBB3-9492
[ticket/11329] Moving colors to colours.css

Moving color values from buttons.css to colours.css

PHPBB3-11329
Merge remote-tracking branch 'remotes/nickv/ticket/9492' into develop

# By Joas Schilling
# Via Joas Schilling
* remotes/nickv/ticket/9492:
  [ticket/9492] Fix missing phpbb_container in unit tests
  [ticket/9492] Fix typo in SQL query
  [ticket/9492] Ensure to delete the avatar/rank data when we change it
  [ticket/9492] Ensure to update all avatar values when the avatar is changed
  [ticket/9492] Fix undefined user_avatar_* values when updating the group avatar
  [ticket/9492] Add unit tests for custom ranks and avatars
  [ticket/9492] Retain custom ranks and avatars when setting users default group
Merge remote-tracking branch 'remotes/cyberalien/ticket/11328' into develop

# By Vjacheslav Trushkin
# Via Vjacheslav Trushkin
* remotes/cyberalien/ticket/11328:
  [ticket/11328] Replace long buttons text with short text
  [ticket/11328] Add language variables for buttons
Merge remote-tracking branch 'remotes/cyberalien/ticket/11329' into develop

# By Vjacheslav Trushkin
# Via Vjacheslav Trushkin
* remotes/cyberalien/ticket/11329:
  [ticket/11329] Moving colors to colours.css