R1:a817aa6c7171

Add an "Abort Older Builds" build step to Harbormaster

Summary:
Ref T13124. See PHI531. When a revision is updated, builds against the older diff tend to stop being relevant. Add an option to abort outstanding older builds automatically.

At least for now, I'm adding this as a build step instead of some kind of special checkbox. An alternate implementation would be some kind of "Edit Options" action on plans with a checkbox like `[X] When this build starts, abort older builds.`

I think adding it as a…