Make modals and popups easier to use with keyboard and assistive tools (#2371)

## Context

Modals can be improved in order to be used properly by everyone.

For now:

- screen reader users don't necessarily understand there are in a modal,
because we don't use any modal dialog-related HTML attributes
- keyboard users can get lost by focusing things behind the modal,
because of one detected bug in our mousetrap pausing system, and because
we don't do anything about trapping the tab key in modals.

## Proposed solution

… Fix everything, noticeably by following the ARIA modal dialog pattern.

## Related issues

## Has this been tested?

- [x] 👍 yes, I added tests to the test suite
- [ ] 💭 no, because this PR is a draft and still needs work
- [ ] 🙅 no, because this is not relevant here
- [ ] 🙋 no, because I need help <!-- Detail how we can help you -->

## Screenshots / Screencasts

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