Make the new Build Plan behavior "Restartable" work

Summary:
Ref T13258. Implements the "Restartable" behavior, to control whether a build may be restarted or not.

This is fairly straightforward because there are already other existing reasons that a build may not be able to be restarted.

Test Plan: Restarted a build. Marked it as not restartable, saw "Restart" action become disabled. Tried to restart it anyway, got a useful error message.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13258

Differential Revision: https://secure.phabricator.com/D20230