[Web] Add system mails (send mails to all mailboxes via LMTP)
[Web] Allow to add more administrators
[Web] Fix domain administrator editing
[Web] Remove some foreign keys
[Web] Remove username from API
[Web] Remove more .php extension from code
[Web] More minor fixes
helper-scripts/check_translations.rb checks if keys present in the master
translation file are used in any php or js code. Writes a sed comand to
stdout to remove unused keys from all lang files.
This saves unnecessary work when translating.
