Closes#1612
Merges @afkbrb work to finish @wetneb protype, including:
* adding support for OAuth in addition to basic auth
* supporting registration of single user Wikidata OAuth consumers.
* adding extensive tests
* cleaning up the login dialog
This depends on the next release of the Wikidata Toolkit, but we're currently publishing our own snapshot that includes the necessary changes (thanks @wetneb!)
Co-authored-by: Antonin Delpeuch <antonin@delpeuch.eu>
