More parser work: fixed a few bugs with [[intlinks]] in headers, a bug that caused the paragraph parser to return an empty string, and added a warning/backup-and-restore for when a render stage returns an empty string.

ALTER TABLE {{TABLE_PREFIX}}users_extra ADD COLUMN date_format varchar(32) NOT NULL DEFAULT 'F d, Y';
ALTER TABLE {{TABLE_PREFIX}}users_extra ADD COLUMN time_format varchar(32) NOT NULL DEFAULT 'G:i';