Helper for more direct access to the API client

Using the helper over APIPage will afford a few benefits, namely that:
1) instantiation and authentication will only occur once per run of the
suite as opposed to with every invocation of `on(APIPage)`; 2) tests
will not have to rely on explicit support for each API-client feature in
APIPage.

Change-Id: Iee7abba399766a2863b7185aa2b74e85ec4f7f3c