Add "delete all" button to notification view

[alanjosephwilliams]

Description

We allow a scrollable list of notifications from each agency. When a notification is read, it remains in the list. To delete a notification, the notification must be opened to reveal the delete button. We should offer the user a way to delete all messages quickly and easily.

Action

• Add delete all button at bottom of notifications view. It should red, but otherwise not prominent, as it is not a desired action most visits to the view.
• At a future date, we may need to consider an automatic deletion or "expiration" of notifications.