Mail digest

Implementation for #278

* daily digests are sent every week day (monday-friday) at 22h. The digest for a user contains up to 10 messages from the user timeline and recommended users.
* daily digests are sent every week day (monday-friday) at 23h (for each user at the anniversary day of his subscription). The digest for a user contains up to 10 messages from the user timeline, recommended users and groups.

Some improvements could be made in another pull request : 

* emails should be sent in html (which would be easier for links and presentation)
* recommended tags could be included 
* statuses could be more cleverly selected when the timeline exceeds 10 messages