Improve Singleton related document actions

[derweili]

Is your enhancement related to a problem? Please describe.

Singleton document types should only have one document. Currently its possible to add multiple documents of a singleton document type via multipel ways:

1. Via the "new document" context menu on the top left

2. Via the "duplicate" document action while editing a singleton.

Its also possible to duplicate or unpublish a singleton which should also be not allowed.

We should hide singletons from the new document context menu and hide unsupported document actions for singleton document types.

Designs

No response

Describe alternatives you've considered

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[derweili]

I already (accidentally) pushed a change for this to trunk

[lcnogueira]

@derweili , just confirming: are you planning to create a PR with the changes you had pushed to trunk b70fa10 ?