Plain Text values don't render as links

Summary: In theory, a Plain Text value is now exactly the same as a single Tag. But it doesn't have one of the most important and useful aspects of Tags -- it doesn't display as a link. (Also, if you go to the named Tag's page, it doesn't list this as a reference.)

Fixing this isn't obviously trivial. I've tried to make Plain Text Type render as a link, but that causes Display Name to break.

It may be that the right fix is to give up on mandating that Tags are always Sets, and instead allow you to have, eg, ExactlyOne Tag. Fixed by doing this.