Link Customer <=> salesman

[erratumsolutions]

Discussed in #77

^{Originally posted by zycabathware May 29, 2022}
Add an option to link a salesman with customer.

1. If a user is adding a customer, then that customer should get assigned to this user automatically
2. When placing a purchase order, this user can only see customers assigned to him
3. Need to implement a Location <=> Area module to handle this