Fix #4982: Add route to remove a post from a favorite group

app/views/favorite_groups/remove_post.js.erb

@@ -0,0 +1,7 @@
+<% if @favorite_group.errors.any? %>
+  Danbooru.notice("<%= j @favorite_group.errors.full_messages.join("; ") %>");
+<% else %>
+  Danbooru.notice("Removed post from favorite group <%= j @favorite_group.pretty_name %>");
+<% end %>
+
+$("#add-to-favgroup-dialog").dialog("close");