Tidy up models
Tidy up models

- Update the Notification and NotificationHistory model to reflect the database.
- Updates to datatypes, removal of indexes and addition of indexes.

Why?
After running the `flask db migrate` command there are many deltas because we did some work to update the notification and notification_history tables, however, the SQLAlchemy models were not updated to reflect those changes. This PR cleans up all those deltas.
However, there are still some differences that can be done but we can look at that in another PR.

alphagov/notifications-api,alphagov/notifications-api