In order to remove the deprecated flag_undefined_default=True
argument, this commit updates the following flags to always be
enabled using a new temporary class:

- course_experience.course_outline_page
- course_experience.unified_course_tab

Adds a temporary setting `USE_DEFAULT_TRUE_NAMESPACE`,
to enable a monitored rollout of this change.

TNL-7061 is the ticket where these flags will actually be
removed. This requires more careful work including removing
all dead code, and potentially refactoring tests that were
testing shared functionality, but only when the flag was
False.

ARCHBOM-1316