Add feature overrides to DVCS webapi tests

Review Request #1382 — Created Sept. 25, 2018 and updated — Latest diff uploaded

Review Board
I had been running all the unit tests for DVCS-related changes with the
`reviewboard.diffviewer.features.dvcs_feature` enabled view
``. This had the side effect of enabling the feature
for unit tests that required the feature to be enabled, but as soon as
the feature was disabled those unit tests would fail. Those unit tests
now have proper feature overrides when they are making API calls.

Testing Done:
Ran the unit tests with the DVCS feature disabled.

Reviewed at