Rewrite the tag counter widget (the one above the tag edit box) to use
React. This is a trial run for using React elsewhere.
We actually use Preact instead of React because it's lighter weight.
We use Mobx for state management because it's cleaner than React's
setState or useState.
This incidentally fixes a couple bugs:
* The tag counter wasn't updated when deleting tags with backspace
* The tag counter wasn't updated when pasting in new tags.
