Test that contributors invited as admins materialize as admins
Failing acceptance test that admins can remove invited admins
Show all contributors on Contributor Management screen
Implement "remove contributor" functionality for admins
Refactor contributor removal controller specs

We adopt the style used in the OrganizationInvitation controller tests,
which only assert that we:

  1. authorize the resource
  2. assert the behavior when a user is and is not authorized to perform
     a given action
Test authorization to remove contributors.
Document and test OrganizationUser#only_admin?
Document OrganizationUser#email
Test that admins can remove non-admin contributors