Decide on proper selecting of wikibase URLs

[arcstur]

The OAuth token we request is only valid for wikidata.org, and not test.wikidata.org for example. If the user wants to point to a different wikibase server, then the authentication should be different. As of know, wikibase url is selected as env var, and not as a dropdown/type when creating a new batch. Maybe we could allow users to select (1) wikidata, (2) test wikidata, or (3) custom server. Our OAuth token could allow editing on (1) and (2), and the user, when selecting (3), should provide its own direct token for us to use when authenticating.