Archive, part 1a-ii: PATCH /api/v1/repos/:owner/:name {archived} sets and clears archived_at for admins #2390

Part of #2341.

Done looks like:

• [ ] {archived: true|false} on the repository PATCH sets and clears the date. It is for admins only, and any other type is refused with 422. Clone and push keep working.
• [ ] Tests: a round trip, a non-admin gets 403, and a clone of an archived repository works.