Modals: add focus auto-focus and focus trapping

[alexmdac]

Modals (Directory Picker, Editor, Version Checker, Usage Detail, etc.) don't
manage focus:

1. Opening a modal doesn't move focus into it.
2. Tabbing can escape the modal to elements behind the overlay.

Per the WAI-ARIA dialog pattern,
modals should auto-focus the first interactive element on open and trap
Tab/Shift+Tab within the modal while it's visible.

focus-trap-react would handle
both concerns with minimal code.