implementing user sign-up restriction. extended config package such that multiple can be on, and added a config package for restricting users.
separating form and view logic more cleanly
Merge branch 'develop' of github.com:learningequality/ka-lite into corrections

Conflicts:
	kalite/securesync/users/views.py
Merge pull request #796 from bcipolli/corrections

HTH: IDOC: Allow option for restricting user sign-up