Enabling External User Management breaks public Signup

Steps to reproduce:

1. Enable External User Management and Public Signup in Confluence Admin > Security Configuration.
2. Log out and attempt to use the "Sign up" link, you will get a blank screen.

The "Signup" feature works fine when External User Management is disabled.

Additionally:

• The feature is named poorly for what it is supposed to do
• The feature doesn't do what it is supposed to do, which is disable adding a user to a group.