Admin should be able to disable or enable users

[fluturecode]

Please run this along the backend PR at 'Shift3/dj-starter#67'

Changes

1. Updated user model
2. Updated user.api
3. Updated userListView
4. Updated userDetailForm

Purpose

To allow Admins to disable/enable all other users.

Testing

1. Pull in the changes to your local copy of this branch and run yarn start
2. Run the backend on branch admin-disable-users
3. Check that you can disable user.
4. Check that the user field is updated after disabling.
5. Check to enable that user.
6. Check that you can disable them again.

Screenshots