change EventDTO, add AddUserEventController, add the search function in UI and backend

src/main/resources/schema.sql

@@ -7,5 +7,6 @@ CREATE TABLE event
     event_id INT PRIMARY KEY AUTO_INCREMENT,
     group_id INT NOT NULL,
     user_id VARCHAR(50),
-    event_payload VARCHAR(255)
+    event_payload VARCHAR(255),
+    visibility BOOLEAN
 );