Add Django debug_toolbar (when USE_DEBUG_TOOLBAR=True); allow INSTALLED_APPS (& more) in local_settings

Background:
Debug-only tools = good!

Changes:
* Allow `INSTALLED_APPS` and `MIDDLEWARE_CLASSES` to be defined in local_settings, will be the start of the tuple-lists in settings.
* Add the Django DEBUG toolbar (DDT) into our code--useful!  Enable by setting `DEBUG=True` and `USE_DEBUG_TOOLBAR=True` in local_settings.py
* Added a decorator which allows adding the DDT to API request responses (@allow_api_profiling), and added this to the coach reports API query.  This is necessary as the DDT only decorates HTML/XML-type markup, not things like JSON.