Add support for Dex to Quay

Fixes #306

- Adds support for Dex as an OAuth external login provider
- Adds support for OIDC in general
- Extract out external logins on the JS side into a service
- Add a feature flag for disabling direct login
- Add support for directing to the single external login service
- Does *not* yet support the config in the superuser tool

templates/ologinerror.html

@@ -11,7 +11,7 @@
         <h2 style="margin-bottom: 20px;">There was an error logging in with {{ service_name }}.</h2>
 
         {% if error_message %}
-          <div class="alert alert-danger">{{ error_message }}</div>
+          <div class="co-alert co-alert-danger">{{ error_message }}</div>
         {% endif %}
 
         {% if user_creation %}