feat: Add setup check to verify whiteboard backend server version and connectivity

[juliusknorr]

This should help us detect a few pitfalls with the setup

• Detect if the Nextcloud app and backend server versions are not in sync
• Detect if the Nextcloud server can properly connect to the whiteboard server
• Detect if the whiteboard server can connect back to the Nextcloud server

The setup check is indicating problems now in the admin overview as well as when running occ setupcheck and when going to the whiteboard admin settings.

Fix #144

Adds some checks for parts of #238

[hweihwang]

When I tested a bit, it kept saying failed to connect to whiteboard server even the whiteboard working fine. Seems the checks doing a curl from nextcloud backend to whiteboard server, not entirely sure but should it be a check from frontend to whiteboard server @juliusknorr (Maybe one check/call from FE => status endpoint of whiteboard server)

[juliusknorr]

• Need to check connectivity issues with docker when the node process is running outside of it