Add combined_skymap to alerts

Once !864 (closed) is merged, we'll need to add the combined skymaps to avro alerts. This means

• Adding a field to the new alert schema (see userguide#284 (closed))
• Adding the skymap to the alert generation logic, both for kafka alerts and GCNs
• Coordinating with kafka brokers (gcn and scimma) to make sure our kafka alerts aren't too large for their configurations. We will probably need a configuration change from at least scimma, as two multi-order skymaps will put us over their max message size of 1MB.