Fix #2109: Recursively unwrap loaders to support template partials

[matthiask]

Description

The current template source view only supports unwrapping one level of
template loaders depending on other loaders.

When using django-template-partials with cached templates we have to
unwrap the loader twice to get to the concrete loaders.

Fixes #2109

Checklist:

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

[matthiask]

Note! The first commit adds the passing test, the second commit adds
the template partials integration (which causes the test to fail) and the
third commit fixes the issue. The change can be squashed, this is just a note for reviewers.

[tim-schilling]

Should we always be testing with template partials? I think with the django-csp tests, we have specific tests that install the package and test against it. That said, this is likely to be a short-time thing. Both django-csp and template-partials are very likely to be merged to core in the 6.x series.

[matthiask]

It does feel a bit bad, I agree. Another way to test this would have been to add a mock loader with a loaders attribute containing the cached loader configuration and testing the panel like that. I thought it easier and more understandable to add django-template-partials as a test dependency than doing the more involved mocking.

We will have to revisit this when template-partials is merged (hopefully), either by switching to a different package which also wraps the cached loader or by adding the relevant testing code to our project.

[tim-schilling]

Thank you for fixing this issue! My other comment isn't a required change, but a discussion topic for us.

[TomHall2020]

Bit late to catch this here, but when I was trying to follow the contributing guide and use make test this was breaking on me as django-template-partials was only added to the tox config, and not to requirements_dev.txt as well