Join, leave, registrations endpoints

todo: remove smb from event endpoint

WebApp/Mapping/EventRegistrationMapping.cs

@@ -0,0 +1,17 @@
+using WebApp.DTOs;
+using WebApp.Entities;
+
+namespace WebApp.Mapping
+{
+    public static class EventRegistrationMapping
+    {
+        public static EventRegistrationDto ToEventRegistrationDto(this EventRegistration er)
+        {
+            return new EventRegistrationDto(
+                er.EventId,
+                er.UserId,
+                er.RegisteredAt
+            );
+        }
+    }
+}