MDLSITE-1958 Notify users on new contribution comments

When a new comment is added to the contribution page, send the
notification to the contribution author and to the maintainers of the
language pack. If the contribution is assigned to a maintainer, on that
particular maintainer is notified. Otherwise, all maintainers of the
given language pack are notified.
MDLSITE-1958 Convert all notifications to the messaging API

Do not send notification on the contribution state change. Do not set
the pop-up notifications by default.
Check for all pending contributions, not only those "In review"

As suggested by Koen.