FEATURE: Salesperson user field

Added a “Salesperson User” field to the customer record.

This allows you to assign an admin user to a customer as their salesperson.

The following mergecodes are available for emails with customer context:

##SALESPERSONUSERID##
##SALESPERSONUSERNAME##
##SALESPERSONFIRST##
##SALESPERSONLAST##
##SALESPERSONEMAIL##
##SALESPERSONEXTENSION##
##SALESPERSONPHONE##
##SALESPERSONALTPHONE##
##SALESPERSONFAX##
##SALESPERSONEMAILNODOMAIN##