Fix #4930: "Show scores" setting should be preserved

Make "show scores" setting persistent.

The setting is stored in a `post_preview_show_votes` cookie. This means
it's remembered on a per-device basis, but not on a per-account basis.
This is so users without an account can use the setting, and so you can
use different settings on desktop and mobile.

The `view=score` URL param has been replaced by `show_votes=true`. The
`show_votes` URL param overrides the `post_preview_show_votes` cookie.

app/views/posts/update.js.erb

@@ -1,6 +1,6 @@
 <% if @post.valid? %>
   var $post = $("#post_<%= @post.id %>");
-  var $new_post = $("<%= j post_preview(@post, fit: :compact, show_deleted: true, show_votes: params[:view] == "score", size: @preview_size) %>");
+  var $new_post = $("<%= j post_preview(@post, fit: :compact, show_deleted: true, show_votes: @show_votes, size: @preview_size) %>");
   Danbooru.Blacklist.apply_post($new_post.get(0));
   $("#post_<%= @post.id %>").replaceWith($new_post);
   <% if params[:mode] == "quick-edit" %>