Improve formality of "HarbormasterBuild" states

Summary:
Ref T13072. Currently, Builds have basic states (like "passed" and "failed") and pending states where a command has been issued but not yet executed (pausing, resuming, restarting, and aborting).

These are handled in a bit of an ad-hoc way, and not everything treats them the same way. In particular, the build page can concurrently report a build as "Aborting" and "Pausing", with different icons and colors.

Make everything use the same logic so that a Build can only be in exactly zero or one pending state, and use the same icons and colors.

Also tighten up which transitions are allowed: for example, it doesn't make sense to pause an aborting build.

The tighter rules don't all produce great UX right now (like "You can't pause this build.", when it would be better as "You can't pause a build which is already aborting." or similar), but just leave that alone for now.

Test Plan: Viewed builds, applied various state changes, ran BuildWorker to effect the state changes, grepped for affected methods, tried to issue various out-of-sequence build commands.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21685