PLT-7302: Aggregate Elasticsearch indexes over a certain age. (#7224)

* PLT-7302: Aggregate Elasticsearch indexes over a certain age.

This is done by a scheduled daily job, in order to keep the shard count
to a sensible level in Elasticsearch.

* Use map[string]string instead of StringMap

config/config.json

@@ -296,7 +296,9 @@
         "EnableSearching": false,
         "Sniff": true,
         "PostIndexReplicas": 1,
-        "PostIndexShards": 1
+        "PostIndexShards": 1,
+        "AggregatePostsAfterDays": 365,
+        "PostsAggregatorJobStartTime": "03:00"
     },
     "DataRetentionSettings": {
         "Enable": false