Allow Overriding admin templates

[benf710]

Problem Statement
In base Django you are able to override admin templates to implement additional customizations. This doesn't work for the templates that are being used for django-simple-history.

Describe the solution you'd like
django-simple-history should allow you to override the base admin templates if needed in the same way that Django does.

The Solution
The way that Django implements this is by passing multiple template names to the templating engine. Django will try them in order until one of them resolves and will use that one.

Additional context
Django Docs: Admin | Overriding admin templates.
Django Admin Source lines