Simple mechanism to show persistent but dismissable notifications

[diosmosis]

It's useful to provide users as much diagnostic information as possible, including notifications for specific problems when we know they are relevant. Once a user solves those problems (or decides it's not a problem), those notifications can be annoying. Unfortunately there's no simple way to make a notification dismissable. This issue can be closed when that mechanism is created.

The flag to hide a notification should probably be stored in local storage rather than in the database.