Bootstrap 5

[inducer]

This is starting to look like something.

• [x] Fix tests
• [x] Starter code fold doesn't work
• [ ] Adapt sample course content: https://github.com/inducer/relate-sample/pull/15
• [x] InlineMultiQuestion forms are broken
• [ ] InlineMultiQuestions have a JS error
• [x] Flow pages still refer to $(...).resizable()
• [ ] Correct/incorrect markings on InlineMultiQuestion are broken.

Known issues (won't prevent merge):

• https://github.com/django-crispy-forms/crispy-bootstrap5/issues/98
• Flow start page is too wide on narrow screens (not new, #893)
• BS5 applies display: block and width: 100% to form elements. This changes how InlineMultiQuestions look, in that it prevents in-line form elements.

Closes: #616