Rename soc to group

[dnlburgess]

• Replace all occurrences of society with group
• users.py: Don't pass a user in the registration form when the user isn't registered yet
• users.py: Grabbing the enable_signups value from the DB and directly testing with .enabled didn't work, create an intermediary variable.
• Make the default value of the sessions JSON field in a Group an empty list
• general.py: when logging out, the index page is now at general.index