Cleanup public_wopi_url and handle internal hostnames

[juliushaertl]

public_wopi_url is not very useful, we should get rid of that and instead always inherit the url from the discovery response. However when collabora connects back to Nextcloud we always pass the URL that a user used to access Nextcloud which in some cases might not be suitable and an internal url should rather be used.

Discovery endpoint returns either the requesting hostname or if configured server_name from coolwsd.xml

Requests and used urls

• nextcloud -> collabora (wopi_url setting) (internal)
• browser -> collabora (derived from /hosting/discovery, request above or server name) (external)
• collabora -> nextcloud (derived from url in the browser) (ideal internal, currently external)
  • public_wopi_url was introduced to set csp for deriving from request
  • if exposing internal url we can have a override config value
    • if not this gets more tricky and we need this to work on the collabora server