Allow Multiple Sites in Purchases

On some purchases, for example extending support/warranty for hardware we may raise a PO which covers multiple sites in a single PO rather than splitting them out to individual PO's per site.  As a business we are tending to do this more where we may have equipment in multiple countries but raising the support extension at the regional HQ.

Without this being able to be set to multiple sites we would have to work around it which could end up with errors in support renewals as we would be unable to define which site a PO would immediately affect without checking on 2 other systems.

We would need the ability to select multiple sites for a PO so that we know the PO itself is covering multiple sites.