feat: Ignore forked repositories unless already configured (#561)

This feature is particularly useful in the case that renovate is enabled on all repositories a user/account has. Many of those might be forks, and it makes no sense to renovate those by default. Instead, Renovate will skip over forked repositories unless a renovate.json has been added to their root.
Also, Renovate will now prune branches after deciding to skip a repository, whether because of a fork or disablement.

Closes #541