Fix CircleCI build not running on personal GitHub repositories.
CircleCI pipelines were not running for personal GitHub repositories when

a review request is made because the GitHub username was being extracted

from the wrong place.
RBIntegrations was pulling the wrong key from extra_data and it's been

updated to grab the associated HostingService's username.
This patch depends on /r/11198 and /r/11204 for the unit tests to pass.

Manual testing: A personal public and private repository was set up and

integrated with CircleCI. They are confirmed to be running without using

Review Board. Then, a review request was made for both the repositories.

Using the CircleCI web UI, the pipelines were triggered.
Unit tests were created to verify that the username is being pulled

from the correct place for the 4 different GitHub repository plans.

These were ran on Review Board 3.0 and Review Board 4.0 and their

supported versions of Python.