Add the Django Debug Toolbar

Summary
=======
- Add the Django debug toolbar to Horizon (http://django-debug-toolbar.readthedocs.org/)
- Add the template profile extension to the debug toolbar (https://github.com/node13h/django-debug-toolbar-template-profiler/)

Motivation
========
Horizon has issues with performance, particularly at scale, but the amount of existing code and moving parts can make these issues difficult to track down, particularly to those new to the project. Extra tooling to highlight the issues should allow us to make better progress.

Description
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Add the debug toolbar and template profiler, with minimal extra config. Collapse by default to make it less intrusive.

UX
===
N/A - UX isn't very important due to non-prod nature

Wireframes, Mocks, Videos and UI Markup
---------------------------------------------------------
N/A

Testing
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N/A

Outside Dependencies
==================
N/A

Requirements Update Required
========================
- 'django-debug-toolbar'
- 'django-debug-toolbar-template-profiler'

Doc Impact
=========
* Will require explanation of new settings