TNL-160 Default cohorts, automatically created.

Please review:
@cahrens or @andy-armstrong 
@gwprice
I think it's important to test
`verify the previous users remain in the default cohort, but new users are in the auto_cohort`

and maybe not as important (since the former implies), but an incremental test for
`verify the existing users remain in the default cohort`.

The reason one may suggest not testing these is because of the current implementation. However, the purpose of the test is to catch any future inadvertent (or on-purpose) changes to the implementation, which would change this behavior.