Start cleaning up all the customization templates

Since Django 1.9 the option to overextend a template has existed, sadly this feature wasn't usable in Horizon until Rocky when our customer template loaders were fixed/updated.

Now that is has been fixed, we should move to building our templates so they are most usefully overextended rather than overridden.

This would get rid of the need for so many random customization templates, and mean we can get rid of them in future in favor of lots of useful blocks in our templates.

We'd add docs to explain how this worked, and encourage deployers and plugin authors to write templates to make use of it.

Also once merged this new documentation in Django itself should be linked to to help explain why this is useful and how to use it:
https://github.com/django/django/pull/9884#issuecomment-382670037