posts: rewrite tag counter widget in React.

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.

app/javascript/packs/application.js

@@ -43,6 +43,7 @@ export { default as PostTooltip } from '../src/javascripts/post_tooltips.js';
 export { default as PostVersion } from '../src/javascripts/post_version.js';
 export { default as RelatedTag } from '../src/javascripts/related_tag.js';
 export { default as Shortcuts } from '../src/javascripts/shortcuts.js';
+export { default as TagCounter } from '../src/javascripts/tag_counter.js';
 export { default as Upload } from '../src/javascripts/uploads.js.erb';
 export { default as UserTooltip } from '../src/javascripts/user_tooltips.js';
 export { default as Utility } from '../src/javascripts/utility.js';