favorites: delete favorites for expunged posts.

Delete favorites that have an invalid post_id because they belong to an
expunged post.

This bug of not deleting favorites after a post is expunged was fixed
long ago, but old favorites were never cleaned up.

Fixes #4711: Some users have incorrect fav count.

script/fixes/085_fix_invalid_favorites.rb

@@ -0,0 +1,10 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+Favorite.transaction do
+  Favorite.left_outer_joins(:post).where("posts.id": nil).destroy_all
+
+  print "Commit? (yes/no): "
+  raise "abort" unless STDIN.readline.chomp == "yes"
+end