Turn on background permissions syncing by default

Created by: unknwon

The background permissions syncing has been shipped since 3.14. https://app.hubspot.com/contacts/2762526/company/1959283693 has been using this feature since April 1st and no news ever hear back about permissions issues, IIRC they have ~7k repositories with GitLab (cc @dadlerj to confirm this is still the case).

Plan to turn on by default after 3.16 branch cut so we have it for 3.17.

Once that's done, create another issue to remove on-demand implementation of permissions and aim for 3.18.