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/directives/ui/header-bar.js

@@ -14,7 +14,10 @@ angular.module('quay').directive('headerBar', function () {
     },
     controller: function($rootScope, $scope, $element, $location, $timeout, hotkeys, UserService,
           PlanService, ApiService, NotificationService, Config, CreateService, Features,
-          DocumentationService) {
+          DocumentationService, ExternalLoginService) {
+
+      $scope.externalSigninUrl = ExternalLoginService.getSingleSigninUrl();
+
       var hotkeysAdded = false;
       var userUpdated = function(cUser) {
         $scope.searchingAllowed = Features.ANONYMOUS_ACCESS || !cUser.anonymous;