Allow issuers to manage claim codes.

Fixes #219.

This is a kind of odd solution to the problem, because it duplicates all the claim-related endpoints under the `/admin/` path to the `/issue/` path, rather than e.g. changing the auth privileges required to access only one set of claim-related endpoints.

@brianloveswords, if you want to reject this PR in favor of a cleaner solution, I totally understand.