Logistration: Add CSRF cookie to combined login/registration form

Small fixup for setting the CSRF when loading forms.  With this PR, we'll set the CSRF cookie:

* When the login/registration form is loaded.
* When a client requests the login/registration form definition.
* When a client requests the password reset form definition.

It removes the CSRF cookie from the POST request handlers, which don't need to set the CSRF token.

@stephensanchez please review.