This patch updates the logic that prevents about:welcome from showing when the Nimbus `aboutWelcome` `showModal` variable is set to true. If separate screens for the modal are provided, it can now be shown on top of the default onboarding flow in about:welcome. A `requireAction` variable that sets the modal to be a window modal requiring action (other than pressing ESC) to dismiss is also added to the `aboutWelcome` Nimbus feature Differential Revision: https://phabricator.services.mozilla.com/D230185