login: Require CSRF token to login

This patch fixes a vulnerable scenario, where sequential login attempts can
be made without an existing session, and without a valid CSRF token. This
scenario lends itself well for brute force password attempts, because
attackers can avoid using a session and still send requests to determine if
a set of credentials are valid. This vector also avoids the authentication
lockout mechanism, because it requires an ongoing session to shutdown the
requests.

This patch addresses the issue by requiring a session and a valid CSRF token
generated by the server and placed in the session to be submitted with the
credentials. Therefore, an existing session and a Cookie header are required
to process a login attempt. Secondly, the CSRF token will be changed on the
server after each login processed. Therefore, for each session, a subsequent
GET request would be necessary before submitting another login attempt.