Eliminated login requirement during InvitationCode validation:

A user who receives invitation email likely doesn't have an account yet, so we shouldn't force them to the LOGIN_URL to verify their code.  Instead, the developer now has the freedom to choose where to send the user (for example, to a registration page).  As part of this update, Anonymous users are now prevented from poaching a private InvitationCode that doesn't belong to them.

Added specificity and docstrings to tests.py

Added a test scenario for public invitation codes
Previously, any user could use any valid InvitationCode, even if the InvitationCode was private. I think this is undesired.