Sort of make Harbormaster build logs page properly

Summary: Depends on D19139. Ref T13088. This doesn't actually work, but is close enough that a skilled attacker might be able to briefly deceive a small child.

Test Plan:

• Viewed some very small logs under very controlled conditions, saw content.
• Larger logs vaguely do something resembling working correctly.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19141