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/partials/common/_search.html.erb

@@ -9,6 +9,9 @@
     <% if params[:view] %>
       <%= hidden_field_tag :view, params[:view] %>
     <% end %>
+    <% if params[:size] %>
+      <%= hidden_field_tag :size, params[:size] %>
+    <% end %>
     <%= text_field_tag("tags", tags, :id => tags_dom_id, :class => "flex-auto", :"data-shortcut" => "q", :"data-autocomplete" => "tag-query") %>
     <button id="search-box-submit" type="submit"><%= search_icon %></button>
   <% end %>