invites: clear e-mails before showing modal.

Otherwise, if you get an error those e-mails are still around the next
time you try to invite someone.

(imported from commit b521a74f4d6c0d67271f804221f519d1aa7551ff)

zephyr/static/js/invite.js

@@ -24,6 +24,7 @@ function reset_error_messages() {
 }
 
 function prepare_form_to_be_shown() {
+    $("#invitee_emails").val("");
     update_subscription_checkboxes();
     reset_error_messages();
 }