Email-like field in /api/v1/accounts/verify_credentials (for PeerTube OAuth plugin and alike)

Addresses pleroma-support#56.

OAuth plugins like PeerTube's demand email field in identity endpoint response (/api/v1/accounts/verify_credentials in case of Pleroma). Yet, it's insecure to output real email (e.g. Twitter and Facebook require OAuth app to explicitly request such feature, and the feature is only granted after app review).

This MR adds fqn field (fully-qualified nickname, i.e. "nickname@host") to response. It should be usable as email substitute by such plugins (without the ability to send emails to it, of course).

Also more documentation on configuring Pleroma as OAuth 2.0 provider is added.