fix #3164: allow one to display all quarantined spam mails

fix #3164: allow one to display all quarantined spam mails

If the API call returned more than one pmail entry, inject an "all"
entry which, if selected, drops the user parameter and loads the
quarantine mails of all users from the backend.

The webinterface has only some issues regarding deselection (all in
the grid header or if we need to deselect due to the search filtering
out some elements) - for that the underlying issue was found and a
widget toolkit patch was provided.

The rest seems now pretty performant, albeit more than a few 100k
mails may become a problem here. But, in such big setups the mail
admin won't tinker to much whith the users mail anyway, if they are
even alowed to do so depending on their jurisdictions privacy laws
and companies privacy policy.

So, basically this is more for evaluation or for smaller setups but
got quite often requested, and as there's not more data
exposed/returned then already available I see no real argument
against it.

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