Provide statistics information about the registry

Summary:
GET /registry/stats query returns information about the registry,
currently the count of repositories.

Ref T1461

Test Plan:
Browse http://localhost:8000/registry/stats

Integration tests provided to count the registry repositories.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1461

Differential Revision: https://devcentral.nasqueron.org/D1868