Fix it so that upvoting or downvoting a revealed thresholded comment
doesn't hide it again.
The fix is to explicitly store a `data-show-thresholded` flag on the
comment, instead of manually hiding elements with jQuery, and to morph
the comment HTML instead of replacing it so that the state isn't lost
after voting. Alpine.js is used for this, which isn't strictly necessary,
but is useful to test the library before adopting it on a wider scale.
https://alpinejs.dev/start-here
Allow users to upvote a comment, then downvote it, without raising an
error or having to manually remove the upvote first. The upvote is
automatically removed and replaced by the downvote.
Changes to the /comment_votes API:
* `POST /comment_votes` and `DELETE /comment_votes` now return a comment
vote instead of a comment.
* The `score` param in `POST /comment_votes` is now 1 or -1, not
`up` or `down.`
* Add comment scores.
* Rework voting buttons so that you can click the upvote/downvote
buttons to toggle votes.
* Hide the edit, delete, undelete, and report buttons behind a popup menu.
* Show the upvote/downvote/reply buttons to logged out users. Redirect
them to the login page instead.
