Use app's frontend version in preview

[nkylstad]

Description

Our preview uses the same app-frontend version for all apps. We should update preview to use the specific version that a given app is set up with, so that f.ex:

• v4 apps show v4 app frontend in preview
• v3 apps show v3 app frontend in preview
• an app that uses the explicit version f.ex. 3.56.3 should be shown this explicit version in preview

We have an endpoint that fetched both app-frontend and app-lib (backend) versjon for the app.

We should be able to fetch the .cshtml-file from the app-repo, instead of our studio-copy. In this way we don't need to fetch the version, just use the file directly.

[nkylstad]

Tested with several different versions of app-frontend. In each case, I could see that the file that was loaded for app-frontend matched the response from the version-api. So this works as expected!