Add an unsubscribe link to notification emails

- create an UnsubscribeToken class that works similar to AuthenticationToken
- create notifications/unsubscribe route to handle unsubscribe action
- add unsubscribe link to mailer with email and raw token for authentication
Merge branch 'devel' into feature/notifation_unsubscribe

Conflicts:
	spec/models/notification_spec.rb
Support other delivery methods in unsubscribe
WIP
Merge branch 'devel' into feature/notification_settings

Conflicts:
	app/models/user.rb
	db/schema.rb
Merge branch 'hotfix/fix_2fa_spec' into feature/notification_settings
Separate one-click unsubscribe and notifications settings

Create a controller just for one-click unsubscribe

Create a settings controller with a notifications page
Create Authorization model and use it to maintain notification settings

Move notification settings to authorization object (one per authorizated user/application pair)
filter for authorizations with notifications scope
remove comments, correct typos
Merge pull request #426 from 18F/feature/notification_settings

Feature/notification settings