Convert common helpers to Typescript (#2541)

js/src/common/helpers/username.tsx

@@ -0,0 +1,12 @@
+import * as Mithril from 'mithril';
+import User from '../models/User';
+
+/**
+ * The `username` helper displays a user's username in a <span class="username">
+ * tag. If the user doesn't exist, the username will be displayed as [deleted].
+ */
+export default function username(user: User): Mithril.Vnode {
+  const name = (user && user.displayName()) || app.translator.trans('core.lib.username.deleted_text');
+
+  return <span className="username">{name}</span>;
+}