quarantines: dock attachement grid to bottom, avoid toggle button

quarantines: dock attachement grid to bottom, avoid toggle button

A separate header and docked at the bottom reduces crowding at the
top and make the whole view(s) a bit clearer.

This also better mirrors where most mail clients show attachments
(e.g., thunderbird) and allows to avoid the extra button (UX benefits
most of the time from reduced inputs, if it doesn't takes away
features).

Make the panel collapsible for the spam and virus quarantine, and
hide the download button there; note that this isn't done for
security purpose but just wasn't requested and some users may do
better if they only decide deliver/delete here and let their MUA
handle attachements.

Disable collapsing in the Attachement quarantine, it doesn't really
makes sense there and we got a maxHeight already anyway.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>