comments: allow swapping votes.

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.`

app/views/comment_votes/create.js.erb

@@ -1,2 +1,6 @@
-var $comment = $("article#comment_<%= @comment.id %>");
-$comment.replaceWith("<%= j render_comment(@comment, current_user: CurrentUser.user) %>");
+<% if flash[:notice] %>
+  Danbooru.Utility.notice("<%= j flash[:notice] %>");
+<% end %>
+
+var $comment = $("article#comment_<%= @comment_vote.comment_id %>");
+$comment.replaceWith("<%= j render_comment(@comment_vote.comment, current_user: CurrentUser.user) %>");

app/views/comment_votes/destroy.js.erb

@@ -1,2 +0,0 @@
-var $comment = $("article#comment_<%= @comment.id %>");
-$comment.replaceWith("<%= j render_comment(@comment, current_user: CurrentUser.user) %>");

app/views/comment_votes/destroy.js.erb

@@ -0,0 +1 @@
+create.js.erb