Add option to choose a template engine

[Foucauld Degeorges]

Context:
We have two template engines in our app - one that is permissive with invalid variables (e.g. accessing a property on None), the other one raises on invalid variables.
Django Admin requires the permissive one as it makes use of the fallback mechanic on invalid variables.

But we prefer for all our other renders (views, emails, PDFs) to be strict on invalid variables. So we want our strict template engine to be the default one, and specify a custom one for the admin.

The idea would be a Django admin setting named something like ADMIN_TEMPLATE_ENGINE_NAME, that sets the using argument of all TemplateResponses returned by admin views.