Fix a regression in accessing the username for the session.
Fix a regression in accessing the username for the session.

My previous optimization to fetching the user resource along with the
session broke the `get_username()` function, which attempted to follow a
now non-existent link. It's been updated to get the expanded user
resource instead and access the username from that.

Testing Done:
Ran `rbt status`. Before, it would crash with an attribute error. After,
it showed me a list of my open review requests.

Reviewed at https://reviews.reviewboard.org/r/6900/

davidt/rbtools,davidt/rbtools,datjwu/rbtools,datjwu/rbtools,halvorlu/rbtools,reviewboard/rbtools,davidt/rbtools,halvorlu/rbtools,haosdent/rbtools,beol/rbtools,reviewboard/rbtools,haosdent/rbtools,haosdent/rbtools,beol/rbtools,beol/rbtools,halvorlu/rbtools,reviewboard/rbtools,datjwu/rbtools