MailFeed{Rule}: refactor all choices in IntegerField instead of CharField (+ 2-steps migrations).

Move all choices to common, to factorize as much as possible with twitter.
Update forms & core template tags.
more read QS in the manager; we're not at Django 1.7.
remove old file.
Switch from django-social-auth to python-social-auth. Move PSA settings to a dedicated file to make common.py smaller.
Damned. Refresh the bad ones, not the good.
version bump for 0.83.
Merge branch 'release/0.83' into develop
For some reason, it works like that, but not before. Go figure.
update some remaining _articles_ → _items_ in template.
version bump for 0.83.1.
Merge branch 'release/0.83.1' into develop