Link prominently to Harbormaster Buildables in build result output

Summary: Ref T8096. We don't currently link to the buildable, which I think contributes to Harbormaster feeling a little scattered.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8096

Differential Revision: https://secure.phabricator.com/D13405