Limit all queries to just the list of repositories in config.yaml

[stuartlangridge]

With this done, it will be possible to run multiple disjoint dashboard sets out of one mongodb instance. However, this may be hard; repo and team dashboards are by definition limited to just stuff about repositories from the config, and queries on an org dashboard would need to grow another query parameter of {repository_url: {or: [{$regex: /ghuser\/repo1name}, {$regex: /ghuser\/repo1name}]}} which isn't too hard, but user queries are difficult; it's hard to know which users are related to which repositories without precomputing it all ahead of time. This isn't a huge problem -- it means extra contributor dashboards get created, but they aren't linked from anywhere, which is inefficient but not huge -- but it does break lists such as "users not in any organisation".