eslint: Fix unicorn/prefer-spread.

This was initially disabled for IE/Babel-related reasons that no
longer apply.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

web/src/compose_validate.js

@@ -141,7 +141,7 @@ export function warn_if_mentioning_unsubscribed_user(mentioned) {
             `#compose_banners .${compose_banner.CLASSNAMES.recipient_not_subscribed}`,
         );
 
-        const existing_invites = Array.from($existing_invites_area, (user_row) =>
+        const existing_invites = [...$existing_invites_area].map((user_row) =>
             Number.parseInt($(user_row).data("user-id"), 10),
         );