Archiving mirrored pages causes inconsistent state

[timobrembeck]

Describe the Bug

When a page, that is mirrored on another page, is archived, this leads to several problems:

1. The mirrored page is not selected in the dropdown anymore (because archived pages are removed from the queryset of possible mirroring targets), which means the first page of that region becomes the mirroring target when the save button is clicked, which is easily overlooked and means potentially unrelated content is inserted as live content
2. The content of the archived mirrored page is still live somewhere, which contradicts the purpose of archiving a page

Steps to Reproduce

1. Mirror a page somewhere
2. Archive the mirrored page

Expected Behavior

Either one of two things should happen:

1. Archiving a mirrored page should not be possible, similar to deleting a mirrored page
2. The live content should be disabled when a mirrored page is archived (in this case we could also allow deleting a page with the same effect)

Actual Behavior

The mirrored page is archived, but still referenced as live content