Use milestoneable to find/create a milestone
Use milestoneable to find/create a milestone

This way we simplify the code and avoid strange cases like `params[:id]`
having an ID which doesn't belong to the current milestoneable.

AyuntamientoMadrid/consul,AyuntamientoMadrid/consul,AyuntamientoMadrid/consul,AyuntamientoMadrid/consul