Additional affected cluster association for journal issues

[dkistner]

What would you like to be added: As an operator I want provide a list of additional affected clusters to an existing journal issue of a Shoot.

The journal issue should be displayed on all linked Shoots cluster via the issue list and in the Shoot detail view.

Why is this needed: Often there are many Shoots are affected by the same issue and the issue should be tracked within the same ticket.

cc @ScheererJ, @mliepold