Additionally correctly handle cookie path for authentication_data
There were two bugs that exposed an interesting case where two discourse
instances hosted across two subfolder installs in the same domain
with oauth may clash and cause strange redirection on first login:
Log in to example.com/forum1. authentication_data cookie is set with path /
On the first redirection, the current authentication_data cookie is not unset.
Log in to example.com/forum2. In this case, the authentication_data cookie
is already set from forum1 - the initial page load will incorrectly redirect
the user to the redirect URL from the already-stored cookie, to /forum1.
This removes this issue by:
* Setting the cookie for the correct path, and not having it on root
* Correctly removing the cookie on first login
3189dab622