Add UserProfile model
Add notification preferences form, view, template and link from account
Implement checking of user notification preferences before send_notification()
Add link to notification preferences at bottom of email templates
Add userprofile creation method tests
Add tests for notification preferences view
Add tests that notification preferences are respected;
also move existing notification tests to the new test case
Add extra UserPagePermissionsProxy methods:
    - can_edit_pages
    - publishable_pages
    - can_publish_pages
Add tests for new UserPagePermissionsProxy methods
Filter notification preferences form fields according to user permissions
Add UserPagePermissionsProxy tests for a user who can access the admin only
Only show the link to notification preferences if the user has preferences to set
Add tests for notification preferences views' being filtered for less privileged users
Changed User.get_profile to UserProfile.get_for_user
Made wagtailusers work a bit nicer with custom user models
Don't filter by owner in publishable_pages
Merge branch 'notification_preferences'