Can't add a tagset term that is already in use as part of another term

If there is a tagset with the existing term "fire truck" it is impossible to add "truck" as it's own term if it does not already exist.

In order to fix, the currently entered text should always be shown as an option in the term results. I believe this is a problem that is inherited from the jquery.tokeninput.js plugin. There have been a few fixes for other use cases that I've seen online, but none of them seem to fit CiviCRM's specific needs, so I think we'll have to override some parts of that file.