Add a model object based version of the journal writer test objects
This refactors the various conversions happening in the journal writer tests in a single place.
When developing this, I noticed that the journal writer would let Content data go through; Make sure we do not let it do that.
Depends on !157 (closed).
Test Plan
updated tox tests to use the new fixture
Migrated from D3004 (view on Phabricator)