Implement build generations in Harbormaster

Summary:
Ref T5932.  Ref T5936.  This implements build generations in Harbormaster, which provides the infrastructure required to both show users the previous states of restarted builds and to allow users to forcefully abort builds (and their targets).

You can view previous generations of a build by adding `?g=<n>` to the URI, but this isn't exposed in the UI anywhere yet.

Test Plan: Ran a build plan with a Sleep step in it.  Reconfigured it for various sleep times and viewed previous generations of the build after restarting it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5932, T5936

Differential Revision: https://secure.phabricator.com/D10321