Move notification related code to a separate directory

Motivation: Make it a compact module, easier to factorize out into
a sepaarate repo later.
DeOOP Notification templates

ReplyTemplate and NotifierTemplates were semi-classes with only a bunch of
static functions collected under a class. Separating the templates in module level
makes these abominations obsolete.
Create reply-notification for newly registered users

This reply-notification-template will be used for the notifier to send replies,
and can be activated/deactivated
Move domain_mailer into notifications
Notifications tab stub

This tab will list the user's notifications.
A new tab is better (even if now it is a one element list) because
works differently then the rest of the account data.
Fetch subscriptions list from backend

The h.auth.local.ProfileController class (which handles account editing)
has now a profile GET call which sends back all subscriptions for an uri
(now a username)
Fix notifier tests

Many were broken because of the refactoring
Add unsubscribe operation

This is used by email links. This operation sets the active state to false
of a notification subscription.
Add unsubscribe link to emails
Migration script

Added a new Alembic migration script

Upgrade:
- Drop user_subscriptions table (we haven't used it anyway)
- Create subscriptions table (with the new structure)

Downgrade: Inverse
Lazy generate missing reply notifications

This is backwards compatibility for existing users.
At login they'll have their notifications enabled.