Move "Metadata Display"-administration interface to backend-theme

[roromedia]

I suggest using the backend-theme for Metadata Display-routes. Currently it is a bit confusing that the user ends up in the front-end after clicking "Admin->Content->Metadata Displays".
It makes perfect sense for routes like "/metadatadisplay/add" where users must see the end-result of their preview in their front-end theme, but for lists like under "/list" it is confusing.

A possible solution is to add the following parameter to the route in format_strawberry.routing.yml:

options:
     _admin_route: TRUE

For consistency it would make sense to change the routes itself to "/admin/content/metadatadisplay/...".