Dump frontend config JSON with an indent

Fixes gh-7857

IPython/config/manager.py

@@ -86,7 +86,7 b' class BaseJSONConfigManager(LoggingConfigurable):'
         else:
             f = open(filename, 'wb')
         with f:
-            json.dump(data, f)
+            json.dump(data, f, indent=2)
 
     def update(self, section_name, new_data):
         """Modify the config section by recursively updating it with new_data.

IPython/html/services/config/manager.py

@@ -8,7 +8,7 b' import os'
 from IPython.config.manager import BaseJSONConfigManager
 
 class ConfigManager(BaseJSONConfigManager):
-    """Config Manager use for storin Javascript side config"""
+    """Config Manager used for storing notebook frontend config"""
 
     def _config_dir(self):
         return os.path.join(self.profile_dir, 'nbconfig')