Have activate user return json

- Change activate user from admin controller to return json
- Test that it returns json
- Remove unnessary test from log_out spec

This commit was created so that when you activate a user through the api
it returns a json response.

app/controllers/admin/users_controller.rb

@@ -198,7 +198,7 @@ class Admin::UsersController < Admin::AdminController
   def activate
     guardian.ensure_can_activate!(@user)
     @user.activate
-    render nothing: true
+    render json: success_json
   end
 
   def deactivate