Makes ARender a Cloud Service

We want to make ARender a Cloud Service, ie: it is not tied to Nuxeo directly, we can upgrade one without breaking the other.

The main subject is to make ARender Previewer OAuth2 authentication flow working with several Nuxeo.
For that, we need to store OAuth2 provider definition in a datastore accessible by the previewer.

The provider will have this information:

• client id
• client secret
• nuxeo uri

The ARender OAuth2 Provider will need an OAuth2 Consumer configured on Nuxeo. Provider and Consumer must share the same client id and client secret.

Thus, the flow from Nuxeo would be:

• Nuxeo redirects user to ARender with the oauth2 client id
• ARender fetch the provider definition for client id and redirects user to oauth2 authorization page
• Nuxeo logs in and redirects the user to ARender with authorization code
• ARender exchanges this authorization code to an OAuth2 access token
• User is logged in ARender