Fix positioning of tag edit dialog box.

Bug: if you open the tag edit dialog with shift+e, close it, then open
it again, it won't be vertically centered when it opens the second time.

Fix: hide the related tags section *before* opening the dialog, not after.
Hiding it afterwards shifts the layout, which uncenters the dialog.

app/javascript/src/javascripts/posts.js.erb

@@ -89,6 +89,8 @@ Post.open_edit_dialog = function() {
     return;
   }
 
+  $(document).trigger("danbooru:open-post-edit-dialog");
+
   $("#edit").show();
   $("#comments").hide();
   $("#post-sections li").removeClass("active");
@@ -149,8 +151,6 @@ Post.open_edit_dialog = function() {
 
   $tag_string.css({"resize": "none", "width": "100%"});
   $tag_string.focus().selectEnd().height($tag_string[0].scrollHeight);
-
-  $(document).trigger("danbooru:open-post-edit-dialog");
 }
 
 Post.close_edit_dialog = function(e, ui) {