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