When authenticating a principal add a synthetic "APIGatewayUser" object
as the request user. This object is not backed by a database object but
does have an id meaning that DRF templates used to render the API views
will correctly identify the authenticated user.

This unpacked somewhat because of a slight break with Django convention
exhibited by this app.

One needs to be _very_ careful with what is imported as a side-effect of
importing the top-level application module. That is because that module
is imported at application configure time as part of configuring the
application. It follows that applications in general are not yet
configured when the top-level application module is imported.

The top-level module here then directly imports some of the
implementation. So now our implementation must not rely on applications
having been configured at import time.

We got away with this due to the simplicity of the application but
attempting to derive from AnonymousUser triggered the problem: trying to
import anything from django.contrib.auth.models threw an exception about
applications not being configured.

The convention for DRF authentication classes is for them to sit in a
submodule named "authentication" within the top-level application. This
is for good reason; trying to have them in the top-level leads to this
sort of pain.

Unfortunately we have users in the wild using
"apigatewayauth.APIGatewayAuthentication" in their settings and so we
need to support that until users are fixed up to use
"apigatewayauth.authentication.APIGatewayAuthentication".

In the meantime, do some nasty hackery to support what was required by
the original issue.

Closes #2