Open shared link from browser in app

• Server @juliushaertl
• Clients: https://github.com/nextcloud/files-clients/issues/9

Idea is that you click on a shared link (maybe got it from mail) and this opens up browser on mobile/desktop.
Browser then detects this and asks to open it in app. It pass
nc://shownode/server:cloud.example.org&shareToken=ASDFWASD&fileId=123
to mobile/desktop.

On client, we check if we can access the file, or have it shown anonymous:

if (same account)
  if (file id exists)
    show internal
  else 
    #show unauth: 
    if query shared token
      check if password and ask for it
    else 
      show error (internal link was clicked, but not shared)
    fi
  fi
else 
  show unauth: query shared token, check if password and ask for it
fi

webdav request on protected link:
user: shareToken
pass: password

Next step:
- add to my cloud (on federated share) within app

Start with the linked nextcloud/files-clients issue and the server-side shared-link and WebDAV behavior described here. Trace how the nc:// URI, share token, password, and file ID would cross from the browser into the mobile or desktop client. Done requires an agreed server/client flow for internal, anonymous, protected, and federated shares.