When a BUR is approved, put it in a `processing` state. After it
successfully finishes processing, put it in the `approved` state. If it
fails processing, put it in the `failed` state.
If approving the BUR fails with a validation error, for example because
the alias already exists or an implication lacks a wiki, then leave the
BUR in the `pending` state. The `failed` state is only for unexpected
errors during processing.