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
Reset the OmniAuth GitHub stub after every test.
Convert CanCan-style view helper to Pundit-style.

Document the Supermarket::Authorization module to explain how to
interface with Pundit.

The Pundit-style view helper for authorization is to pass the resource
being authorized into the policy method and call the action predicate.

For example to check if the current_user can create a new user, you
would do:

  policy(user).create?