posts: eliminate unnecessary favgroup query on posts show page.

Eliminate a SQL query loading the post's favgroups if the current user
is anonymous (and therefore can't have favgroups).

app/components/post_navbar_component.rb

@@ -23,6 +23,8 @@ class PostNavbarComponent < ApplicationComponent
   end
 
   def favgroups
+    return [] if current_user.is_anonymous? && favgroup_id.nil?
+
     favgroups = FavoriteGroup.visible(current_user).for_post(post.id)
     favgroups = favgroups.where(creator: current_user).or(favgroups.where(id: favgroup_id))
     favgroups.sort_by do |favgroup|