Create an index on harbormaster_buildlog to reduce page load times

Summary: Resolves T5895. This reduces page load times significantly when looking at builds.

Test Plan: Viewed a build, saw the page load a lot faster.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5895

Differential Revision: https://secure.phabricator.com/D10286