Ability to unsubscribe from notifications

Big PR, sorry for that.

Contents:
- notifier code refactoring (100% backend)
   - All notifier related code (including the domain_mailer) was moved into a separate directory (notification) (bit similar to auth). Will be very easy to factor out into a separate repo later.
   - DeOOP notification templates. The previous solution used classes with ugly static methods. (As @tilgovi said in the past it was overkill). Got rid of them. Every notification template has its own module.
   - Introducing Subscriptions table instead of the UserSubscriptions table (was formerly part of the auth module)
   - notifier fully uses subscriptions table
- Notifications tab for account settings
   - The user accounts now has a notification tab, which displays the notifications and the user can activate/deactivate them.
   - And the backend actually sets the active flag for them. (as @tilgovi suggested)

What's left:
- style the notifications tab
- put the deactivate notification url at the end of the generated emails
- rebasing and squashing the commits and write proper commit messages for them
- add new test cases
- write the migration scripts (which adds the reply notification for all existing users)


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No, because I only fetch the active subscriptions here:  https://github.com/hypothesis/h/blob/unsubscribe/h/notification/reply_template.py#L128