posts: make thumbnail size setting persistent.

Make setting the thumbnail size persistent.

The setting is stored in a `post_preview_size` cookie. This cookie can
be overridden by the `size` URL param, like so:

  https://danbooru.donmai.us/posts?tags=touhou&size=180

The `size` param is mainly for testing different sizes without setting a 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: params[:size]) %>");
+  var $new_post = $("<%= j post_preview(@post, fit: :compact, show_deleted: true, show_votes: params[:view] == "score", size: @preview_size) %>");
   Danbooru.Blacklist.apply_post($new_post.get(0));
   $("#post_<%= @post.id %>").replaceWith($new_post);
   <% if params[:mode] == "quick-edit" %>