When an entity on another repo is referenced by using a non-local EntityId in an EntityIdValue (or using a property defined on another repo in a statement), we should track this via the usage tracking mechanism we are already using on the client wikis.

This requires two changes:

1. per-repo usage tracking
2. usage tracking on a repo

Number (2) can be ignored if we assume that in order to use foreign entities from a given repo, the local repo must be a client of that other repo. That seems to be a reasonable requirement, and may help to avoid confusion.