[change] Disallow changing configuration backend from UI

I think we should disallow changing the configuration backend from the UI in:

• device
• template
• VPN server

The work required to make this work flawlessly is too much and the gains are negligible. Users should simply delete the object and create a new one if they need to use a different configuration backend.
In exceptional cases users could use the django shell or scripts to migrate from a backend to a different one (this could happen if they create a custom backend and then they want to switch to it), therefore we should only prevent this from the UI but not add logic to prevent changing the backend directly using the django model API.

Start by locating the UI forms or views for devices, templates, and VPN servers, then trace how each configuration backend field is handled during editing. Done means the UI disallows backend changes for all three object types while direct changes through the Django model API remain possible; check the relevant UI behavior with tests if available.