Add an internal service ref panel to repository "Storage" information

Summary: Ref T13611. Currently, the "writable" property on service bindings has no effect because of a trivial bug. Provide more information in the UI to make this kind of problem observable.

Test Plan:
Viewed "Storage" section of management UI, saw a more-obvious problem with ref management (a non-writable ref is listed as writable).

Maniphest Tasks: T13611

Differential Revision: https://secure.phabricator.com/D21575