Validate SIP username

Mantis: 3891
Show Contract (equals Cusomer-) id

For better identification of the current contract in details/edit views.
Mantis: 3885
Correctly identify customer contracts

the product handle was compared to the class name by mistake
Filter Contract Contacts by whether they have a reseller_id

in case the product in billing_mappings is one of
(SIP_PEERING, PSTN_PEERING, VOIP_RESELLER)
Fix duplicate showing of contracts under Reseller Details - Reseller Contract

The problem occured when a reseller is linked to multiple billing_mappings
Mantis: 3855#c23785