Elements behind modals can still be focused #1679

Describe the bug

When a modal (dialog box) is open, buttons, links and multiselects that aren't part of the modal can still be focused on (using that Tab key) and activated (using the Enter key).

Steps to reproduce

1. Click "Create a project" in the user menu
2. Use the Tab key to move focus to the end of the modal
3. Continue to press Tab

Expected behavior

Focus would "loop back" to the start of the modal, or it would "tail off" and some other part of the browser UI would gain focus.

Screenshot