add Custom Scores panel to the Spam Detector

add Custom Scores panel to the Spam Detector

with this panel, users can manually override a rule score for
SpamAssassin rules. This can be useful sometimes, e.g. if
a certain rule always triggers in a certain environment which
is considered spam by the SpamAssassin rules

after adding/editing a rule, we show diff, similar to the network
panel, and offer a button to apply those changes (and restart
pmg-smtp-filter). the changes can also be reverted

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>