Towards #3552: Add bootstrap5-jinja theme

[clayadavis]

Pull Request Checklist

• [x] I’ve read the guidelines for contributing.
• [ ] I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• [x] I tested my changes.

Description

Towards #3552, here's a light Bootstrap 5 refresh of the bootstrap4-jinja theme.

I'm happy to also create the corresponding Mako theme, but as it's my first contribution, would somebody mind taking a look before I tackle that other one?

[Kwpolska]

I'm sorry, but I have to reject this pull request. Our built-in themes are provided in both mako and jinja versions - but the base version is mako, and the jinja version is auto-generated with the scripts/jinjify.py script. This means that your work would be overwritten when we run the auto-generation script.

I started some work on the Mako version of this theme a while back (but never got around to finishing it, sadly). I'll publish my branch shortly (hopefully later today) and when that is done, I'd be glad if you could help or offer feedback.

[Kwpolska]

FYI, my pull request is at #3572 (the Jinja version will be created after the Mako version is complete). I’d love a review or suggestions on it.