Document IEvent.properties, fix IWelcomeScreenLoad

IEvent.properties is a placeholder that needs to be overriden by extenders for type validation to take place.

IWelcomeScreenLoad should have had properties declared for it. Because it didn't, a faulty call using it was possible.

src/components/views/auth/Welcome.tsx

@@ -76,6 +76,6 @@ export default class Welcome extends React.PureComponent<IProps> {
     }
 
     componentDidMount() {
-        getAnalytics().trackAnonymousEvent<IWelcomeScreenLoad>("welcome_screen_load", { foo: "bar" });
+        getAnalytics().trackAnonymousEvent<IWelcomeScreenLoad>("welcome_screen_load", {});
     }
 }