Add a very basic standalone view for build logs with a "Download Log" button

Summary: Depends on D19132. Ref T13088. This implements an extremely skeletal dedicated log page with a more-or-less functional "Download Log" button.

Test Plan: Downloaded a recent log. Tried to download an old (un-finalized) log, couldn't. Used bin/harbormaster write-log to get a convenient standalone link to a log.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19133