Support OAuth 2.0

[diegodlh]

The plugin currently asks for username and password to login to Wikidata each time an edit batch is requested. Eventually, these may be saved to local storage if desired by the user (#45).

Ideally, plugin should support logging in using OAuth (see https://meta.wikimedia.org/wiki/Talk:Wikicite/grant/WikiCite_addon_for_Zotero_with_citation_graph_support#OAuth). However, OAuth 2 would be needed, because the client is running on the user's computer and can't keep OAuth 1's consumer secret safe.

OAuth 2 is not yet supported by wikibase-edit.

[diegodlh]

Refer to MediaWiki's OAuth 2 documentation for developers: https://www.mediawiki.org/wiki/OAuth/For_Developers#Authorization_2