Organisation admins are not able to assign permissions to other users

[chrisala]

To reproduce:

1. Login as a user that has the "admin" permission on an organsation
2. Go to Admin->Project Access
3. Add, edit and remove a user permission.

Expected:
The admin user can assign and remove the "admin" and "editor" permission to/from other user users for that organsiation.

Actual:
A 401 is returned, preventing the user from making changes.