Site Migration: Refactor module activation during cleanup

[sixhours]

From D148186-code via @daledupreez

At some point, we should refactor how we approach module activation during post-migration cleanup to avoid modules potentially being missed or not activated properly on the site due to plugin conflicts.

Have a dedicated wpcom_site_migration_activate_jetpack_modules() job that specifies a clear set of modules that ought to be activated AND works out what all the plugins are on the site (via something like wp plugin list --status=active --format=csv --field=name), and then runs wp jetpack module activate <modules> --skip-themes --skip-plugins=<all_plugins_except_jetpack>.

[nelsonec87]

Related diff D149308-code

[sixhours]

This was deployed in D149308-code 🥳