PLT-7694: Allow setting a prefix for Elasticsearch indexes. (#7500)

config/default.json

@@ -314,7 +314,8 @@
         "PostIndexReplicas": 1,
         "PostIndexShards": 1,
         "AggregatePostsAfterDays": 365,
-        "PostsAggregatorJobStartTime": "03:00"
+        "PostsAggregatorJobStartTime": "03:00",
+        "IndexPrefix": ""
     },
     "DataRetentionSettings": {
         "EnableMessageDeletion": false,