Store classification in the alert packet

[JulienPeloton]

We compute the classification after the fact to allow dynamical definition of it. But we construct anyway an HBase table ztf.class based on this classification each night -- and all services rely on this static definition of it. Therefore, there is a confusion on the classification tag v:classification (dynamical, recomputed at each query) and classification label used for queries.

We should abandon this idea of dynamical classification to reduce the confusion, and store the classification in the alert packet.