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/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(