This patch makes a few modifications: * Migrates `TranslationsState.neverTranslateSites` (session) to `TranslationsBrowserState.neverTranslateSites` (global browser) * Behavior change: Can no longer update page settings when this list changes, because we have limited info on the session. * Fetches `neverTranslateSites` information initially on app startup. Differential Revision: https://phabricator.services.mozilla.com/D208031