MT#6841 Sound Set Customer section conditional

only when pbx is enabled
or a customer was already set, giving the opportunity to unset it.
MT#7051 set webusername to NULL if empty

because this does not trigger the unique constraint
(webuser_dom_idx) and it makes sense, because one cannot
login with an empty webusername anyway.
MT#3955 API callcosts: GET
MT#3955 API callcosts: filter by subscriber_id
MT#7119 API order_by for collections

most collections support ordering by the column names of
their primary table now
MT#7119 fix broken soundfilerecordings collection

when auto returns false, the catalyst chaining is aborted
MT#7119 API document sortable columns

for collections