invite 5.x-1.7

New features:
- #84120 by knseibert: Added multiple invites option to the invite form.
- #126231: Prepopulate registration form with email from invitation.
- #54935: Persist individual message that gets mailed.
- #124711: Displaying a message with a link to the user's profile when trying to invite a user that's already registered.
- #124906 by ray007 and MarcoR: Added german translations.
- New validation system and error messages to better handle large amounts of emails (based on the patch by knseibert).

Bugs fixed:
- Invitation accepted message a) is no more shown to anonymous users to reduce db load, and b) is only shown when the user has been approved.
- #127681: Fixed deleting of pending invites.
- #128939 by mailleux: Fixed PostgreSQL support.
- Added (non-critical) permission checks at several locations.
- #129979 by NathanRAFT, knseibert: Fixed email displayed in _invite_check_messages().
- #125589: Prevent poedit from whining.
- #132828: Prevent array_merge() from whining.