Improve setup checks and url handling with separate callback url

[juliusknorr]

Improve handling of different URLs. From now on we will allow to configure:

• wopi_url Used by Nextcloud to connect to Collabora in the backend
• wop_callback_url Passed to collabora to connect back to Nextcloud (optional, determined from the browser URL if not set)

The public_wopi_url which was only partly working is no longer ment to be manually set and will be overwritten depending on the /hosting/discovery response.

Further this PR improves:

• Add setup check on occ and admin page
• Give proper error on individual failures of the setup check
• Display configured and detected URLs to make setup issue debugging easier
• Refactor services to have a cleaner structure for setup checks, discovery and capabilities fetching
• Add frontend connectivity check in the admin settings

Note: One thing we cannot check as of right now is if the Collabora server is properly configured. This could be achieved continuing the implementation effort on CollaboraOnline/online#4353

Fix #3313
Fix #3262
Fix #1497
Fix #1835

occ command

occ rich:setup -w "http://collabora:9980" -c "https://nextcloud.local"
✓ Set WOPI url to http://collabora:9980
✓ Set callback url to https://nextcloud.local
Checking configuration
🛈 Configured WOPI URL: http://collabora:9980
🛈 Configured public WOPI URL: https://collabora.local
🛈 Configured callback URL: https://nextcloud.local

✓ Fetched /hosting/discovery endpoint
✓ Valid mimetype response
✓ Valid capabilities entry
✓ Fetched /hosting/capabilities endpoint
✓ Detected WOPI server: Collabora Online Development Edition 23.05.5.4

Collabora URL (used for Nextcloud to contact the Collabora server):
  http://collabora:9980
Collabora public URL (used in the browser to open Collabora):
  https://collabora.local
Callback URL (used by Collabora to connect back to Nextcloud):
  https://nextcloud.local

Screenshots of new admin settings hints

[juliusknorr]

@joshtrichards I'm unsure if you have a development setup, but I'd be curious about your feedback around this given that you did quite some issue triaging efforts. This PR aims to introduce one additional setting but the core part is to improve error reporting for admins. Maybe you have a way to give it a try? Or just give some feedback on further improving the error messages?

[Sprinterfreak]

Tested on unicode domain using server 27.1.4. With commit 4aecf93 applied, richdocuments doesn't break nextcloud anymore.

Fixes #3086 again

[juliusknorr]

/backport to stable28