add Attachment Quarantine

add Attachment Quarantine

the Quarantine part is mostly copied from VirusQuarantine and adapted
the AttachmentGrid is inspired by the SpamScore list

this adds a new quarantine type 'Attachment' in the gui, where the admins
can review and control the mails in the attachment quarantine

it also shows a list of attachment/parts and a link where they can
be downloaded (in case the admin want to only forward a single
attachment or review the file in detail)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Reviewed-By: Stoiko Ivanov <s.ivanov@proxmox.com>
Tested-By: Stoiko Ivanov <s.ivanov@proxmox.com>