Enable Template Caching

Summary
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To improve the template rendering speed of Horizon, by caching templates in memory.

Description
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We should alter the template loading so that it uses `django.template.loaders.cached.Loader`.

- https://docs.djangoproject.com/en/1.8/ref/templates/api/#django.template.loaders.cached.Loader
- https://docs.djangoproject.com/en/1.9/ref/templates/upgrading/

UX
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The end result should be a speedup

Wireframes, Mocks, Videos and UI Markup
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N/A

Testing
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This is tricky, and I could use suggestions. We'd need to research common issues with the cached template loader, and ensure all our templatetags are thread-safe.
https://docs.djangoproject.com/en/1.8/howto/custom-template-tags/#template-tag-thread-safety

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

Requirements Update Required
========================
N/A

Doc Impact
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Release notes need to mention settings alterations, as well as issues that the cached loader may potentially cause.