[FIX] account: prevent selecting company's contacts for manual account.move.line

Only the commercial partner (parent company) should be set in partner field of
an account.move.line.
Fixes #4261, opw 618763
[FIX]stock: return of 'To Be Invoiced' picking should also be invoiced

When creating a return picking, the default invoice state is 'To Be Invoiced' if
returned picking was invoiced. However if the invoice of the picking has not
been generated yet (state '2binvoiced'), the return should also be invoiced.
Fixes #4002