Use mail.backends.filebased on mirror instances
Merge branch 'master' of i44vm3:/srv/praktomat/2014_SS_Mirror/Praktomat
attestation_list: Show attestations_by_others only for tutors, show all attestations to trainers

(working towards #201)
Trainer can edit and withdraw other's attestations

and the mail sent correctly tells who did what. Users and attestations
owners (if different) are notified. This works towards #201.
Now the Trainer may edit all solutions from the regular view

This fixes #201.