Media library should show radios instead of checkboxes if only one item can be selected

Problem/Motivation

In reviewing #2994699-71: Create a CKEditor plugin to select and embed a media item from the Media Library → , @webchick pointed out:

It's a little odd that checkboxes are presented here (indicating multiple selections are possible), but your choices are limited to only one. Radio buttons would be the standard UI element for this kind of choice. However this is a "pre-existing condition" with any time the Media Library is showing you a group of things you are limited to only being able to select 1 of, so fine to handle in a follow-up (and just "normal" priority; there are other visual indicators like the disabling of controls and the fading out of the other selections and the tiny, tiny text in the lower-right saying "0 of 1 selected" to help you understand that you can only select one thing).

Changing those checkboxes to radio buttons if only one item can be selected would probably be a usability (and maybe even accessibility) improvement, but it's out of scope to fix in that issue. So, let's do it here!

Proposed resolution

Figure out if switching to radios for a single-selection situation would be feasible and desirable. And if it is, implement it.

Remaining tasks

Determine if we even want to do this, and what usability concerns may exist.

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD