Confusing UI on Notifications page

### Problem you are trying to address with this feature
The UI for turning email notifications on and off is confusing. This comes out of a ticket sent to zendesk today: https://hypothesis.zendesk.com/agent/tickets/2142. To sum it up: " the fact that there is no "Save" button (or indication of the change being successful) makes for a confusing UI. The page should automatically save and refresh when you check or uncheck the box, but it's a very quick/subtle effect that probably goes unnoticed."

### Your solution
Add a Save button or add a banner to notify the user when changes have been made

No implementation files, tests, or entry points are mentioned. Start by locating the Notifications page and reviewing the existing checkbox save and refresh behavior. Define whether the completed change is a Save button or a visible success banner, then verify that users can tell their notification changes were saved.

Written by the indexing model from the issue text.