This patch allows the extensions success link to also be opened when the migration wizard is running in the spotlight. The issue seems to be that the browser element used in `OpenAboutAddons` doesn't exist when running in spotlight, so I've opted to instead allow the event to be handled with `OpenURL` when running in the aboutwelcome bundle, and also adding a parameter to handle trusted links with OpenURL.
Original Revision: https://phabricator.services.mozilla.com/D213666
Differential Revision: https://phabricator.services.mozilla.com/D214182
