FIX: Don't cause exceptions due to rename of reply_id column

This syncs the value of the `reply_id` column into the `reply_post_id` column until all servers have been deployed and the post migrations ran.

Follow-up to ab07b945c2

db/post_migrate/20200117174646_make_post_reply_id_column_read_only.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class MakePostReplyIdColumnReadOnly < ActiveRecord::Migration[6.0]
+  def up
+    Migration::ColumnDropper.mark_readonly(:post_replies, :reply_id)
+    DB.exec("DROP FUNCTION IF EXISTS post_replies_sync_reply_id() CASCADE")
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end