favgroups: fix favgroups with invalid post ids.

Fix favgroups containing post ids that don't exist. Most of these are from
typos or from old expunged posts that weren't properly removed from favgroups.

script/fixes/060_fix_favgroup_posts.rb

@@ -0,0 +1,12 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+favgroups = FavoriteGroup.select("*, unnest(string_to_array(post_ids, ' '))::bigint AS post_id")
+favgroups = FavoriteGroup.select("*").from(favgroups).where("NOT EXISTS (SELECT 1 FROM posts WHERE id = post_id)")
+
+FavoriteGroup.transaction do
+  favgroups.each do |favgroup|
+    favgroup.remove!(favgroup.post_id)
+  end
+end