Add deluge option to change web UI default password

[ibizaman]

The web UI config file is in web.conf. The password if in the field pwd_salt and is the sha1 encoding of the password.

The issue is this file is also used for other things so we can't just use a symlink to the nix store. We need to merge the field into the existing file.

See https://forum.deluge-torrent.org/viewtopic.php?t=33047