MT#4025 Implement management of customer sound set

If a sound set has a contract_id, only show pbx group (consider it
being a customer sound set).
If it only has a reseller_id, show everything else except pbx group
(consider it being a system sound set).
MT#4025 Fence sound set for subscriberadmins.

Only admins, resellers and subscriberadmins are allowed to manage
sound sets. For subscriberadmins, they have to have a pbxaccount to
do so.