Add option to auto-login after registering

[zpchavez]

Set autologin=1 in the query string to have the user logged in after submitting registration.

See https://github.com/synapsestudios/oidc-platform/blob/master/api/src/plugins/openid-connect/grants/password.js for how to retrieve a token given a username and password.

It may also be useful to update the registration API endpoint to allow auto login and the return of a token. This will allow auto-login to be done on custom forms without using the password grant type.

Tasks

• Update user registration endpoint to redirect to the redirect_uri with the token in the query string if autologin is set.
• When autologin is not set, include a registration_success=1 value when redirecting to login and pass that value to the login template. Update default login template to display a "registration successful" message.
• Update documentation
• Update registration API endpoint. If include_token is set to true, include the auth token in the response.
• Somehow we need to create a session cookie that identifies the user as being logged in. After being auto-logged in, the user should be able to visit OIDC pages such as email-settings and change-password.