Lamb 0.13.0-rc2

Originally written on lamb-releases: This pre-release builds on 0.13.0-rc1 with a large round of correctness and hardening fixes across posting, feeds, import/export, and the admin. All changes below are new since rc1. Upgrade notes Lamb now checks at startup that the PDO driver for your database is actually loaded, and stops with a clear message if it isn't. Before upgrading, make sure the matching extension is installed — pdo_mysql for MySQL/MariaDB, pdo_sqlite for SQLite.