[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
I don't think this is needed, since a couple of lines before you've checked the `storage` against `builds`.
If you want to keep it, I'd change the places of those two checks. Besides it's worth screaming at user that he/she should not use cancel for something different than builds, in that case this check still doesn't make sense.