Add a "registration_source" field to track where users registered from.

This will be used more for registrations that happen programatically via
the API, so we can better distinguish those users.
Fix migration so it actually saves.
Initial CSV download for stats feature.
Update elasticsearch index names to include environment.
Use registration_source for determining unique users during create.
Merge branch 'master' into gsa

Conflicts:
	app/controllers/accounts_controller.rb
	app/controllers/admin/api_users_controller.rb
	app/controllers/api/api_users_controller.rb