Allow synchronization of private repos if owner has AllowUserExternalServicePrivate tag

Created by: asdine

Currently, private repos are sync'ed only if the externalService.userMode setting is set to 'all'.

https://sourcegraph.com/github.com/sourcegraph/sourcegraph@f820a7e2354f5d25fa6e9ed9be13bc4232337323/-/blob/cmd/repo-updater/repos/syncer.go#L240-243

We should allow private repos owned by users that have the AllowUserExternalServicePrivate tag to be sync'ed as well.