Add an endpoint to return active authentication providers

This endpoint can be used by Angular to figure out what authentication
options to display to the user. It returns a dictionary of configuration
details that the front-end needs for each provider.

lemur/static/app/angular/app.js

@@ -18,7 +18,7 @@ var lemur = angular
     'angular-clipboard',
     'ngFileSaver'
   ])
-  .config(function ($stateProvider, $urlRouterProvider, $authProvider) {
+  .config(function ($stateProvider, $urlRouterProvider, $authProvider, AuthenticationService) {
     $urlRouterProvider.otherwise('/welcome');
 
     $stateProvider
@@ -27,17 +27,18 @@ var lemur = angular
         templateUrl: 'angular/welcome/welcome.html'
       });
 
-    $authProvider.oauth2({
-      name: 'example',
-      url: 'http://localhost:8000/api/1/auth/ping',
-      redirectUri: 'http://localhost:3000/',
-      clientId: 'client-id',
-      responseType: 'code',
-      scope: ['openid', 'email', 'profile', 'address'],
-      scopeDelimiter: ' ',
-      authorizationEndpoint: 'https://example.com/as/authorization.oauth2',
-      requiredUrlParams: ['scope']
-    });
+    AuthenticationService.get_providers().then(function (active_providers) {
+      var provider_names = [];
+      for (var key in active_providers) {
+        if (active_providers.hasOwnProperty(key)) {
+          provider_names.push(key);
+        }
+      }
+
+      for (var i=0; i < provider_names.length; i++) {
+        $authProvider[provider_names[i]](active_providers[provider_names[i]]);
+      }
+    }
   });
 
 lemur.service('MomentService', function () {

lemur/static/app/angular/authentication/login/login.js

@@ -12,6 +12,7 @@ angular.module('lemur')
     $scope.login = AuthenticationService.login;
     $scope.authenticate = AuthenticationService.authenticate;
     $scope.logout = AuthenticationService.logout;
+    $scope.get_providers = AuthenticationService.get_providers;
 
     UserService.getCurrentUser().then(function (user) {
       $scope.currentUser = user;

lemur/static/app/angular/authentication/login/login.tpl.html

@@ -3,8 +3,8 @@
   <div class="login">
     <div class="row">
       <div class="col-xs-12 col-sm-12 col-md-12">
-        <button class="btn btn-block btn-default" ng-click="authenticate('Example')">
-          Login with Example
+        <button class="btn btn-block btn-default" ng-repeat="(key, value) in get_providers()" ng-click="authenticate(key)">
+          Login with {{key}}
         </button>
       </div>
     </div>

lemur/static/app/angular/authentication/services.js

@@ -6,6 +6,10 @@ angular.module('lemur')
   .service('AuthenticationService', function ($location, $rootScope, AuthenticationApi, UserService, toaster, $auth) {
     var AuthenticationService = this;
 
+    AuthenticationService.get_providers = function () {
+        return AuthenticationApi.one('providers').get();
+    };
+
     AuthenticationService.login = function (username, password) {
       AuthenticationApi.customPOST({'username': username, 'password': password}, 'login')
         .then(