Implement SSO auth flow

[rijkvanzanten]

No description provided.

[rijkvanzanten]

The flow:

1. The application will redirect to the API on /sso/<provider>
2. The API redirects to the provider (e.g. GitHub/Facebook)
3. The user logs in on the providers website
4. Provider redirects back to the API
5. The API checks the authentication
6. The API redirects back to the application using the referral domain from the initial step 1 request
   The API adds the data to the query string. If it's an error, it's going to be in the ?error parameter, if it's successful, it will add the token in the token parameter

The main problem to solve: where do we store the initial referral domain from step 1 during the login process on the providers website. At this point, the API will be idle.

[wellingguzman]

We will be adding another step to the flow:

7. Instead the token from step 6, to be an actual access_token, it will be a "request token" that will be used to request the actual Access Token from the API.

[rijkvanzanten]

There are a couple of errors with the SSO still:

<br />
<b>Fatal error</b>:  Class 'Directus\Authentication\Sso\Provider\GitHub\Provider' not found in
<b>/var/www/directus-demo-api/src/core/Directus/Application/CoreServicesProvider.php</b> on line
<b>771</b>
<br />

[wellingguzman]

It was an issue with the namespace as because we changed the directory to be lowercase in #99