Keyboard navigation not properly trapped within welcome page reminder/critical error/important update notification modal

[ghostwords]

The following modal overlays do not properly trap keyboard navigation:

https://github.com/EFForg/privacybadger-mv3/blob/9bf7e13f5b29765c665d989b5666397e3ef21ca6/src/skin/popup.html#L36-L73

To see the welcome page reminder modal overlay (div#instruction-text):

1. Add PB to a browser
2. Wait for the welcome page to appear
3. Close the welcome page
4. Navigate to any website, such as example.com
5. Open PB's popup (by clicking on PB's icon, either on the browser toolbar, or inside the "puzzle piece" icon on the browser toolbar)

Once the overlay is open, navigating with Tab should cycle between the close icon and the "CTA" (call to action) button.

The bug is that continuous tabbing leaves the overlay and continues into the rest of the UI (grayed out and partially obscured by the modal).

This follows up on #3140.

Related to #3105.