Exclude forks from all statistics

[stuartlangridge]

GHCrawler on default settings fetches data for all forks of our core repos. Ignore all such data everywhere. (Repository dashboards already ignore it because they explicitly query for the repo name we care about, but contributor and root dashboards don't.)

(Ideally this would be fixed by not fetching that data at all, once #4 is implemented.)

[jeremy-lq]

After a chat with @stuartlangridge I am moving this to milestone .2. While it is an issue, the scope is constrained and well understood, and the fix entails not only a significant refactor but also some upstream GHCrawler work.