This commit allows users to use the trix editor to send
messages with basic formatting and attachements.
* add active storage
* add actiontext
* add richtext field to messages
* add imageprocessing for message attachements
* add html email layout and adjust translations to use html urls
Messagegroups are a new kind of group. Every user can join and leave such a
group on her own. They are useful to create topics for mails.
E.g. if there is a weekly newsletter we can create a new messagegroup for
it and every user which is interested in receiving the newsletter can join
the corresponding group.
This new view sorts the messages by threads based on the reply_to.
Now we store only the message which started a new thread in the reply_to
field to avoid recursive queries.
