Bugfix: Caching issues with mail settings

The code no longer uses a custom API call. Instead all the logic is
handled on the client side.
Merge branch 'feature/email_notification_ui' of https://github.com/reciprocity/ggrc-core into feature/email_notification_hooks

Conflicts:
	src/ggrc/views/notification.py
Merged conflicts with latest changes for Notification settings UI