Add mappings for UserLimit

Overview:
We currently don't map USER_LIMIT by default. However, GOBI does provide the user limit information for electronic orders in the PurchaseOption section of XML...

        <PurchaseOption>
          <VendorPOCode>EBS</VendorPOCode>
          <Code>3U</Code>
          <Description>EBSCOhost - 3 User</Description>
          <VendorCode>EBS3U</VendorCode>
        </PurchaseOption>

The relevant information actually appears in 3 of the 4 fields in some shape/form... The 3U Code means 3 users. The description spells it out, and the VendorCode is the combination of VendorPOCode and Code.

Acceptance Criteria:

• The 5 character code (e.g. EBS3U) should be mapped into the Instructions to Vendor field of the Vendor accordion of the PO Line. If there is already data in the Instructions to Vendor field (which would have come from the GobiAPI OrderNotes field), place the 5 character code before that data, followed by a space, e.g. EBS3U Please rush.
• Do not map anything into the user limit field of the PO Line (so no need to resolve the integer vs text situation). Since this is an optional field, we'll leave it blank for now, and wait to see how the ongoing ERM/License work develops.
• API Tests are updated