Implements blueprint extensible-architecture.
Implements blueprint improve-dev-documentation.
Implements blueprint gettext-everywhere.
Implements blueprint sphinx-docs.

Complete re-architecture of the dashboard to transform it from a standalone django-openstack app to a Horizon framework for building dashboards. See the docs for more information.

Incidentally fixes the following bugs:

Fixes bug 845868 -- no PEP8 violations.
Fixes bug 766096 -- the dashboard can now be installed at any arbitrary URL.
Fixes bug 879111 -- tenant id is now controlled solely by the tenant switcher, not the url (which was disregarded anyway)
Fixes bug 794754 -- output of venv installation is considerably reduced.

Due to the scale and scope of this patch I recommend reviewing it on github: https://github.com/gabrielhurley/horizon/tree/extensible_architecture

Change-Id: I8e63f7ea235f904247df40c33cb66338d973df9e