Make it possible to create an authentication backend of type autologin

refs #6434
Prefer switch rather than if - elseif
Make it possible to edit an authentication backend

refs #6434
Merge remote-tracking branch 'origin/bugfix/rebuild-form-builder-5525' into bugfix/autologin-backend-form-6434
Make it possible to remove an authentication backend

refs #6434
Add AutologinBackendForm

fixes #6434
Merge branch 'bugfix/rebuild-form-builder-5525' into bugfix/autologin-backend-form-6434
Adjust createbackend-action to suit the new backend form interface

refs #5525
Adjust removebackend-action to suit the new backend form interface

Drops additionally the ConfirmRemovalForm of the monitoring module
as there is already one in icingaweb2's form-set.

refs #5525
Remove isExistingBackend/-Instance

It is more efficient to access configuration files only once instead of
accessing them multiple times in different locations.

refs #5525