Newsservice / mass messaging service

This implements a mass message service (#164). This will be expanded at a later stage to include user messages, and to display sent messages on a user overview dashboard instead of reading them only by email.

This isn't fully done, and some naming inconsistencies exist (message vs mass message vs news service), which partly stem from where this will be heading to (see also #164). 


Main features:

* Add mass message controller, which allows users with the right
permissions to send emails to all members of selected instances.

* If users have chosen to receive messages from instance instance
administrators, they will receive them as email.

* Add Message and MessageRecipient models, which store mass messages and
to which these have been sent, in order to display them in a reworked
user dashboard at some point.

Helper features:

* Add permission checks for sending mass messages.
* Add user option: receive mass messages from administrators
* Add default noreply email configuration

Yet missing:

* Every mass message email should contain a link which deactivates
receiving mass messages via email.