[MM-23535] Add disable database search flag and return empty posts results if set (#14245)

* [MM-23535] Add disable database search flag and return empty posts results if set

* Add UpdateConfig function for the SearchStore and hook it into the app lifecycle

* Add the config listener in the server instance instead of using FakeApp

* Instantiate searchlayer as a pointer to avoid passing around copies of it

store/searchlayer/user_layer.go

@@ -61,6 +61,7 @@ func (s *SearchUserStore) Search(teamId, term string, options *model.UserSearchO
 			return users, nil
 		}
 	}
+
 	mlog.Debug("Using database search because no other search engine is available")
 
 	return s.UserStore.Search(teamId, term, options)