Read htgroup files

[ejucovy]

The library should be able to read .htgroup files as well as .htpasswd files. Upon successful login it would look through any .htgroup files that have been configured in settings.py and assign local Django auth.Groups to the user.

Open questions:

• Should it create these groups if they don't exist?
• Should it assign groups to the user on every login, or only on first login (i.e. if the user itself is being newly created)?

Possibly the different behaviors here could be configurable settings.