Feature/auth gui

I still have to clean this code, but it's almost completely functional.

Almost because there are some special characters that do not work in the SRP algorithm.

This pull request contains auth'ed user certificate and anon certificate downloads, ready to be used by your openvpn code by fetching them from Shared Preferences.

It also updates provider.json when a provider is chosen, and builds login/logout gui options only if allow_registration is true inside that file.

Auth'ed user certificate is stored along with its rsa private key in the same Shared Preference.