Playwright tests

[pamelafox]

Purpose

This PR adds playwright tests. Notably:

• Instead of using pytest function-style tests, I used Django unittest-class style tests instead, since the existing tests were already using that style. They should still be runnable with pytest, however.
• It's not possible to run playwright browsers inside Dev Container on an M1 in emulation mode (qemu). They do run in headless mode in Codespaces (not headed), so I made a note recommending that instead. I wasn't able to run playwright codegen inside Codespaces, so I ran that locally to help me generate the tests.
• I also made a few accessibility improvements based off Accessibility Insights checks.

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[X] Other... Please describe: Tests!

How to Test

Follow instructions in the testing page of the lab to run Playwright tests.

[pamelafox]

@kjaymiller Playwright for you!