[1.x] Add dependancy-less modal focus trapping

[calebporzio]

This PR adds "focus-trapping" to the modals in Jetstream.

This means that pressing "Tab" and "Shift+Tab" will only cycle through focusing elements inside the modal.

Here's a gif:

[calebporzio]

Ah, just saw you closed the last one @taylorotwell. Feel free to close this if you don't want it, just figured I'd weigh in.

I imagine you'll get other requests for this from people concerned with accessibility.

It's actually one of the things most modals don't get right in applications and including it inside Jetstream would be adding a lot of value to applications.