related tags: fix related tag section being hidden.

Fix the related tags section being completely hidden when it was
collapsed. The `.hidden` CSS class that was used by related tags
conflicted with the `.hidden` utility class added in 8841de68ac.

app/javascript/src/javascripts/related_tag.js

@@ -133,7 +133,7 @@ RelatedTag.toggle_tag = function(e) {
 
 RelatedTag.show = function(e) {
   $(document).trigger("danbooru:show-related-tags");
-  $("#related-tags-container").removeClass("hidden").addClass("visible");
+  $("#related-tags-container").removeClass("collapsed").addClass("visible");
 
   if (e) {
     e.preventDefault();
@@ -141,7 +141,7 @@ RelatedTag.show = function(e) {
 }
 
 RelatedTag.hide = function(e) {
-  $("#related-tags-container").removeClass("visible").addClass("hidden");
+  $("#related-tags-container").removeClass("visible").addClass("collapsed");
 
   if (e) {
     e.preventDefault();

app/javascript/src/styles/specific/related_tags.scss

@@ -1,7 +1,7 @@
 div#related-tags-container {
   &.visible a#show-related-tags-link,
-  &.hidden a#hide-related-tags-link,
-  &.hidden div.related-tags {
+  &.collapsed a#hide-related-tags-link,
+  &.collapsed div.related-tags {
     display: none;
   }
 }