Notification center internal working rework

[moorscode]

The notification center currently stores the data of notifications in user options.
This means that if a notification is removed but no unregistered, the notification will still remain present.

This causes problems in several cases:

• Addons throw notifications - the addon gets disabled
• Upgrade routines - notifications are removed in a specific version

Suggested change:

• Instead of saving the data of a notification, just store the ID and the state of muted/dismissed

If a notification/state of the site is present or not is checked on every request.
A refactor on where notifications exist sounds like a good addition. These should not have to live in separate contexts. If they are bundled together in a "Site Health" class the overview of which notification exist and when they are triggered would be much more transparent.

Needing some eyes/approved approach from the architects team (cc @atimmer / @herregroen)