Store info about last successful subscription trigger

[Christoffer-Cortes]

Description

Information about last successful subscription trigger should be stored. Either together with failed notifications in a general notification collection or in a separate collection.

Motivation

It is unclear whether a subscription has been triggered in the GUI.

Exemplification

The failed notifications endpoint looks like this:
/failed-notifications?subscriptionNames=Sub1,Sub2,Sub3
So the suggestion then is that last successful could look something like this:
/last-successful-notifications?subscriptionNames=Sub1,Sub2,Sub3

Benefits

The user can see that their subscription is working.

Possible Drawbacks

[henning-roos]

This issue needs to be updated with correct information.