Ahead of T275470: Add option to allow specific user groups to vote to 'Basic options' list, we need to use a TagMultiselectWidget in an HTMLForm. This currently isn't possible, and it's easy to imagine that it could be useful for more use cases than just our current one.
What needs to be done:
- Implement HTMLTagMultiselectField
- Make MediaWiki's PHP widget TagMultiselectWidget instantiable (it's currently abstract; there isn't an OOUI PHP TagMultiselectWidget)
- Make a small wrapper JS widget for OO.ui.TagMultiselectWidget that adds the hidden field needed for HTMLForm submission