Add support for disabling user registration.

This provides a checkbox in the General Settings page for disabling user
registration. This only takes effect when using standard authentication,
and will hide the "Register" link and redirect requests made to
/account/register/ to the /account/login/ page instead.

Tested the /account/register/ path and the existence of the link with the following combinations:

* Registration enabled and standard auth
* Registration disabled and standard auth
* Registration enabled and LDAP
* Registration disabled and LDAP

All cases work. Note that LDAP is a good enough test for non-standard auth types, since the only auth-specific checks are against "builtin".