mod_base: Fix remember password for Chrome

Added class "z_logon_form" to the logon form, and let its target be a
hidden iframe. When a z_logon_form is posted, we post to this hidden
iframe, to trick chrome into showing the "save password" option. The
logon form submit event cannot be cancelled because that blocks the
form-remembering.

Fixes #491
(cherry picked from commit 94f909aa485033518154ccd52db0ab866db31d62)