Info mailing

A system to send info mails to participants should be added. Info mailings can be created independently of events, with a subject and a RichTextField.

On the event detail page (cf. #13 (closed)), mailings can then be triggered. Mails are sent to participants separately, and Paweljong needs to record which mailing was sent to which persons. The mailing is idempotent, meaning that it can be triggered multiple times, and on subsequent sends, only participants that have not been mailed before are mailed.