This patch adds and changes a couple of things in onboarding: - Adds the account login screen as the final screen to be displayed to the user if they are not logged in to a mozilla account, while the gratitude screen remains the final screen if the user is already signed in to a mozilla account - Adds the SVG image used in the account login screen - Adds fullscreen to all of the onboarding screens via the `fullscreen` property - Replaces the addons picker screen which is not fullscreen compatible with the amo introduce screen, which is fullscreen compatible Differential Revision: https://phabricator.services.mozilla.com/D236050