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/javascript/src/javascripts/posts.js

@@ -25,6 +25,7 @@ Post.initialize_all = function() {
   if ($("#c-posts").length && $("#a-index").length) {
     this.initialize_excerpt();
     this.initialize_gestures();
+    this.initialize_post_preview_size_menu();
   }
 
   if ($("#c-posts").length && $("#a-show").length) {
@@ -241,6 +242,19 @@ Post.toggle_relationship_preview = function(preview, preview_link) {
   }
 }
 
+Post.initialize_post_preview_size_menu = function() {
+  $(document).on("click.danbooru", ".post-preview-size-menu .popup-menu-content a", (e) => {
+    let url = new URL($(e.target).get(0).href);
+    let size = url.searchParams.get("size");
+
+    Cookie.put("post_preview_size", size);
+    url.searchParams.delete("size");
+    location.replace(url);
+
+    e.preventDefault();
+  });
+}
+
 Post.view_original = function(e = null) {
   if (Utility.test_max_width(660)) {
     // Do the default behavior (navigate to image)