[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
Exchange the whole body with simple:
```
return build.Status == buildapi.BuildStatusPending || build.Status == buildapi.BuildStatusRunning
```