Numerous keyboard accessibility issues in the media browser

Here's a patch that pulls in the patch from 🐛 Make modal iframe tab accessible Needs review and additionally:

1. Removes hidden TAB'able items from the modal
2. Shows a highlight when focused on the image thumbnails
3. Allowing selecting and de-selecting images from the keyboard

keshav.k → made their first commit to this issue’s fork.

Merged!

Automatically closed - issue fixed for 2 weeks with no activity.