[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
If you've created a separate flag for build canceling you should check its existence here as well. If somebody adds different flag your code will still remove the build which I guess, is not desired.