Improve setup checks

[juliusknorr]

• Check discovery/capabilities on the backend and return a proper status with additional details if something failed (connection issue/timeout)
• Check protocol to be equal between discovery response and the requesting URL of the admin page Fix: missing settings messages #2095
• Validate discovery response in the frontend, so that Collabora can be reached in the browser
• Have a way to validate connectivity from Online to Nextcloud through Check endpoint to validate WOPI configuration from integrators CollaboraOnline/online#4353

@kendy I think it would be quite nice if we could have some kind of validation endpoint in online that would perform wopi operations during setup with a test file and report back possible error messages that might have happened. Then we could directly show the user a hint that e.g. if Collabora could not reach Nextcloud when configuring. What do you think about that?

[juliusknorr]

Started hacking on a check endpoint on the collabora side - WIP at CollaboraOnline/online#4353