add dismissal buttons to our notifications

[johncowen]

Description

Follow up of #3566

We need the dismissal actions to not only track when you have dismissed a notification and store that, but also remove that storage once the logic that showed the notification is reversed.

We also want to "garbage collect" the local storage values every-so-often so notifications appear again after say 7 days (you'd have to dismiss again, or actually sort out the issue to prevent it appearing again)

[johncowen]

This is milestone-d 2.10.x for now, but may slip