Move code over to new siteconfig_settings context manager.

Review Request #10184 — Created Sept. 26, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
b3bab5c...

Reviewers

release-4.0.x had added an override_siteconfig context manager for unit
tests, and then later release-3.0.x got a very similar one using the more
Django-like name siteconfig_settings. This change moves the former
over to use the latter.

Ran unit tests.