Support oAuth2 Authorization Code Grant flow

[Guts]

During the refactor to publish the version 3, only the oAuth2 Legacy (Resource Owner Password Credentials Grant Type) flow has been implemented to allow solve migrations.

The Backend (Client Credentials Grant) flow has been restored for the 3.2 version (see: #74).

It's time to add the user authentication used in the most of Isogeo web apps based on Isogeo API: the WebApp (Authorization Grant) flow.