Reset users' startpages if referenced stream/dashboard is deleted. #2695
This change adds a listener and the necessary events, which are posted
Motivation and Context
As described in #2400, when a user has set a dashboard/stream as start page and that entity is deleted, the user gets an error every login/click on the Graylog logo on the upper left.
Types of changes
This change adds a listener and the necessary events, which are posted when a stream or dashboard is deleted. The listener will then reset any user's startpage if it references the deleted entity.