* Required kid be a header claim as according to spec
https://tools.ietf.org/html/rfc7515#section-4.1.4 (JWT is an extension of JWS)
* Updated error message for missing kid
* Fixed admin-api key unit tests
* Fixed regression and acceptance tests
refs #9865
- small refactoring to make both session and admin api key handling similar
- admin api key authentication is still disabled, but easy to enable
- added proof test how to authenticate using admin api keys
refs #9865
- the outer authentication layer wants a consistent interface of each authentication package
- admin.authenticate
- session.authenticate
- furthermore, there is no need to put the full feature into the exposed function name
refs #9865
- Added `auth.authenticate.authenticateAdminApiKey` middleware
- accepts signed JWT in an `Authorization: Ghost [token]` header
- sets `req.api_key` if the token is valid
- Updated `authenticatePrivate` middleware stack for v2 admin routes