Allow to register accounts from the setup wizard

[juliushaertl]

The client should be able to connect to the Nextcloud registration app in order to create an account right from the client user interface, similar to what some XMPP clients already do for public servers.

Existing discussion about registration API:
nextcloud/server#211
nextcloud/registration#41

Users should get logged in automatically after they registered, with a newly gernerated app password, similar to the new flow authentification: https://github.com/nextcloud/server/pull/4479/files#diff-652646310472be421d6b80289b907cb8R235

[juliushaertl]

API proposal: nextcloud/registration#41 (comment)