Actually use the Jinja2 template backend

[matthiask]

Refs #1881.

Description

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Fixes # (issue)

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[tim-schilling]

I pushed this forward for the next person. Here's my message from the last commit:

It instruments the single template render, but not the inherited
templates and I'm guessing not the included templates either.
I suspect we're going to have to patch jinja templates more robustly
than relying on the django jinja backend template class.

[matthiask]

@tim-schilling It would be great if we were able to inspect all of Jinja2's rendering but maybe that's harder because of Jinja2's template compilation, or what do you think? I didn't do much research here, so I'm basically just speculating.

I think what we have with your changes is already much better than the status quo. We could merge this, document the limitations and move on?