Remove debugging

Signed-off-by: Michal Čihař <michal@cihar.com>
Make email validation work without cookies

We still use session to store the data, but the session ID is kept in
the URL, so we can recover it (if the session is still valid).

Fixes #518

Signed-off-by: Michal Čihař <michal@cihar.com>
Better notice about confirmation link validity

Warn users that the link has limited validity (it mostly depends on
configured session validity and session backend).

Issue #518

Signed-off-by: Michal Čihař <michal@cihar.com>