Drop support for the Django template language in export templates

[jeremystretch]

Proposed Changes

Export templates currently support either the Django or Jinja2 template language (which can be toggled on a per-instance basis). This issues proposes to drop support for Django templating and support only Jinja2.

Justification

As the Jinja2 language is significantly more powerful than Django's built-in language, we can remove it to standardize on only Jinja2 (which is already the case for other models, such as custom links).