Convenience method to get multiple permissions
Convenience method telling whether a group fulfills any given permission
Allow change of from email and name in lib.mail.to_mail and friends
Implement news service (#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.
Expose mass message feature in instance admin UI
Fix migration script
Make InstanceController._get_current_instance a classmethod
Move massmessaging in instance settings context to massmessage controller

See discussion in #259.
Merge pull request #1 from nidico/instance-settings-message-in-massmessage-controller

Instance settings message in massmessage controller
Consistent naming of Massmessage (vs. MassMessage)

see discussion in #259.
Fix docstring
Fix migration script
Use PEP 8 conforming empty list check
Fix single value check
Require global.message if not massmessage not used in instance context
Adjust massmessage sender address options

Use `adhocracy.email.from` instead of `adhocracy.email.noreply`, and
only offer this option if the system configuration allows this.

Furthermore add an explanation on why a certain sender option is
disabled.
Restrict messaging by permission

Users with permission `global.message` may now send messages to users
from all instances, whereas users with permission `instance.message` may
only send messages to users from authenticated instances.
Restrict instance.message to admins by default
Add note about preconditions to receive news service mails
Fix need_login check

This now not only applies if a permission is missing, but also if *not
logged in* is the explicit cause for the authorization denial.