Add conversion infrastructure with LQT API source.

Modular design means we can read from the DB, an XML dump, a talk page
or any other potential source of historical Flow posts if we want to.

Supports history and headers.

Bug: 45088
Change-Id: If41b45908542bf68521c0b31388ca6d33fd818f0
Merge "Add conversion infrastructure with LQT API source."