pools: drop post_count column (fix #3667)

db/migrate/20181108205842_drop_post_count_from_pools.rb

@@ -0,0 +1,10 @@
+class DropPostCountFromPools < ActiveRecord::Migration[5.2]
+  def up
+    remove_column :pools, :post_count
+  end
+
+  def down
+    add_column :pools, :post_count, :integer, default: 0, null: false
+    Pool.update_all("post_count = cardinality(post_ids)")
+  end
+end