Add authentication decorators

[jb3]

Right now the authentication route will set an admin property to True in the token if a record exists in the admin collection that looks like:

{
  "_id": "112233445566778899"
}

Let's write a couple of decorators for use across the application:

1. Validate that tokens provided in the Authorization header are legitimate (signed by us), else return an error.
2. Validate that the authentic token has admin set to true, else return an unauthorized error.