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

store/sql_supplier.go

@@ -786,6 +786,8 @@ func (me mattermConverter) ToDb(val interface{}) (interface{}, error) {
 	switch t := val.(type) {
 	case model.StringMap:
 		return model.MapToJson(t), nil
+	case map[string]string:
+		return model.MapToJson(model.StringMap(t)), nil
 	case model.StringArray:
 		return model.ArrayToJson(t), nil
 	case model.StringInterface:
@@ -809,6 +811,16 @@ func (me mattermConverter) FromDb(target interface{}) (gorp.CustomScanner, bool)
 			return json.Unmarshal(b, target)
 		}
 		return gorp.CustomScanner{Holder: new(string), Target: target, Binder: binder}, true
+	case *map[string]string:
+		binder := func(holder, target interface{}) error {
+			s, ok := holder.(*string)
+			if !ok {
+				return errors.New(utils.T("store.sql.convert_string_map"))
+			}
+			b := []byte(*s)
+			return json.Unmarshal(b, target)
+		}
+		return gorp.CustomScanner{Holder: new(string), Target: target, Binder: binder}, true
 	case *model.StringArray:
 		binder := func(holder, target interface{}) error {
 			s, ok := holder.(*string)