Check nextcloud app compatibility somehow

With #1631 (closed) we saw that Nextcloud apps can be upgraded to incompatible versions without us noticing. We should have a system in place to prevent that. Some possibilities:

  • Parse the app compatibility list (I think this is available as a json file from the Nextcloud app store, though that might not contain all our apps) and check against that.
  • Check the list of disabled Nextcloud apps at runtime against a known list of allowed ones.
  • Add tests for Nextcloud to test app-specific functionality.
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