Feature/source permissions

[lukavdplas]

Changes permissions for contributors to be based on groups (close #160)

• Adds a ContributorGroup model to track cohorts of contributors. (As discussed, I did not use Django's Group model here because managing student assistants and being able edit admin site permissions should be different privileges.) Each group has access to its own set of source texts.
• The is_contributor boolean field is removed: users are contributors if they are added to a contributor group.
• Mutations related to a source text now require that the user is a member of an appropriate contributor group. If not, the mutation request will be rejected.
• Some queries now have an optional parameter editable to view only data you have permission to edit. This is now applied in the Data entry page, so users only see the sources they can edit.

The checks for mutations are not very DRY and create a lot of bulk in the code. Perhaps I'll still look for some abstraction there.