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

static/js/pages/signin.js

@@ -8,7 +8,12 @@
     });
   }]);
 
-  function SignInCtrl($scope, $location) {
+  function SignInCtrl($scope, $location, ExternalLoginService, Features) {
     $scope.redirectUrl = '/';
+
+    var singleUrl = ExternalLoginService.getSingleSigninUrl();
+    if (singleUrl) {
+      document.location = singleUrl;
+    }
   }
 })();