Allow the inclusion of an "autoTrigger" attribute on AboutWelcome SingleSelect components `content` prop`. When set to true, the action for the tile with the id matching `content.selected` (if no value provided, defaults to the first tile in the list) is automatically triggered when the tile renders. Because this deviates from our usual patterns for triggering actions, a guard is included to ensure this can only be used for sidebar prefs (with room to expand for other actions/prefs in the future). Differential Revision: https://phabricator.services.mozilla.com/D223422