feat: add renovateFork option

99% of the time we should not Renovate forked repositories. Previously, we skipped onboarding them unless a renovate.json was present. However, that meant that if someone forked a repository where the source was using Renovate, then the fork gets renovated automatically too. After this change, forked repositories need to set renovateFork=true in their renovate.json

Closes #940

BREAKING CHANGE: Forked repositories now need to configure renovateFork=true in renovate.json