When a user tries to regsiter while logged in, just send them home

Summary: This error message is pointless and dead-ends logged-in users needlessly if they're sent to the register page by documentation or Advanced Enterprise Sales Funnels.

Test Plan: Visited /auth/register/ while logged in, was sent home.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15739