Use custom URL

[jornane]

It should be possible in the app to connect by entering a custom URL. At this moment this is not so easy to implement because the geteduroam flow requires three URLs. This should be taken into consideration for the design of discovery v2. In v2 I want the discovery to point to a single JSON file that lists all three URLs (and possibly more metadata).

I'll make this issue here now, so we remember we want this feature but that the new discovery is a dependency.