Make category optionnal and hide it from the form

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

js/src/types/event.model.ts

@@ -53,7 +53,7 @@ export interface IEvent {
   title: string;
   slug: string;
   description: string;
-  category: Category;
+  category: Category|null;
 
   beginsOn: Date;
   endsOn: Date;