Send emails to a group of users

[spaenleh]

Send a message to a group of users.

Select which users:

• all verified users of the platform
• all verified users of the platform, active in the defined time frame
• members of an item
• a single user

Add the message, potentially in many languages

A background process handles sending the message, by sending it to each user individually.

There is the possibility to inspect the sending process and see which emails may have failed.

[spaenleh]

Part of this has been implemented in #48