[web/desktop] Make media overlay non-draggable

Summary:
ENG-4655

According to this issue, -webkit-app-region: drag (which is used for header dragging of the window) eats all events even for elements which defined outside of the header and just overlap with it.

The fix is to apply -webkit-app-region: no-drag to the multimedia overlay (we already apply it to the wordmark and navigation arrows).

Test Plan:

• Tested if clicking on the black overlay closed the image, both in inside header and outside
• Tested if close button closed the overlay, and if the cursor pointer correctly changed when hovering
• Tested if window wasn't draggable with overlay, and if it was draggable without overlay
• Tested if navigation arrows still worked (without overlay)

Reviewers: kamil, tomek, inka, patryk

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8848