In Harbormaster, record byte length on the build logs

Summary: Depends on D19135. Ref T13088. Denormalize the total log size onto the log itself. This makes reasoning about the log at display time easier, and we don't need to fish around in the database as much to figure out what we're dealing with.

Test Plan: Ran bin/harbormaster rebuild-log, saw an existing log populate. Ran bin/harbormaster write-log, saw new log write with proper length information.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19136