Expose User.can_manage_workers in RetrieveUser

Closes #1847 (closed)

I cleaned up the unit tests in arkindex.users.tests.test_registration in 79bf1196 because they were not testing the RetrieveUser response, so adding can_manage_workers led to zero changes to do in the tests, which is scary. The diff specific to can_manage_workers is in 11e00774