Add a missing field to the mappings for modelSnapshot documents (elastic/elasticsearch#396)

This fixes one of the problems of elastic/elasticsearch#394

Original commit: elastic/x-pack-elasticsearch@ea627767d2

elasticsearch/src/main/java/org/elasticsearch/xpack/prelert/job/persistence/ElasticsearchMappings.java

@@ -636,6 +636,9 @@ public class ElasticsearchMappings {
                 .endObject()
                 .startObject(Quantiles.TYPE.getPreferredName())
                 .startObject(PROPERTIES)
+                .startObject(Job.ID.getPreferredName())
+                .field(TYPE, KEYWORD)
+                .endObject()
                 .startObject(ES_TIMESTAMP)
                 .field(TYPE, DATE)
                 .endObject()