Font Library: adjust warning when a font is in use and is about to be deleted

[annezazu]

Pulling from the FSE Outreach Program's Final Touches call for testing:

However, I found that the activation/deactivation and deletion of fonts somewhat confusing. Why would fonts deletion be offered on active, check marked, fonts? Would it make more sense to only offer fonts deletion to the inactive (unused) ones? Because deleting an active font directly impact the pages that use them, which will cause the page to automatically fall back to the default font. Besides, this is done without any warnings.

Currently, if you delete a font, you're met with this warning:

This warning doesn't mention or change if a font is currently in use. I think it would be advantageous to clarify that during the deletion process.