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
Remove duplicate code
Move adhocpy from developtools.cfg to adhocracy.cfg

Having adhocpy available is also useful in production, e.g. to execute
helper scripts.