Create Bonafide: auth/register

Refactor SRPAuth, SRPRegister ProviderBootstrapper and maybe ProviderConfig into a separate package: "Bonafide":https://leap.se/en/docs/design/bonafide The pixelated team already started his own package to tackle this separation need: https://github.com/pixelated-project/leap_auth/

(from redmine: created on 2015-05-15, closed on 2016-09-30, relates #7550 (closed))