Add option to disable Rspamd password through secure_ip option

[crimist]

https://github.com/anonaddy/docker/blob/1be033be30231b28b0c7ec2d9585a0c586806d37/rootfs/etc/cont-init.d/14-config-rspamd.sh#L95

This is useful in instances where one has their own auth in front of rspamd.

I'm thinking adding a RSPAMD_DISABLE_WEB_PASSWORD boolean which sets secure_ip = 0.0.0.0/0. This is backwards compatible and very explicit to the user that they are setting a potentially dangerous option.