From Rocky we can now support Django's recursive template
inheritance. Let's move away from all these stand alone
templates for deployers to override and instead direct them
to recursively extend the existing templates and the blocks they
need.
This adds more blocks, docs on how to use them, and an example
theme which can be turned on and used to show how this works.
blueprint: less-customization-templates
Change-Id: I69f1e16ff1b88cec78580df0911fe3c01b7507dd