[WIP] Add option to cancel a build

this is a rough draft, I'm actively working on it.. so it may throw errors and it may have dirty code.

   - Add cancelling and cancelled status for builds
   - Remove pod and update builds to cancelled when a cancel event was triggered for the build
`Canceled` - once you've set it, you can't unset it, so the effect to an end user is a transformation (so past tense is appropriate).