Add a UI element for reviewing older generations of Harbormaster builds

Summary:
See PHI446. Ref T13088. Currently, there's no way to access older generations of a build unless you know the secret ?g=1 URI magic.

When a build has multiple generations, show a history table and let users click to see older run information.

This is currently very basic. It would be nice to show when each generation started, who started/restarted it, and what the build status was at the time the build was restarted. There's currently no convenient source for this information so just add a bare-bones, working version of this for now.

Test Plan:
Viewed pending, single-run and multi-restart builds. Saw table on builds with more than one generation. Clicked table entries to see different build data.

{F5471160}

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19217