Rename current BillingAccountContact helper class

This issue is part of the creation and implementation of the BillingAccountContact model to cache the "Bill to" and "Sold to" Zuora contacts:

1. Rename current BillingAccountContact helper class 👈 You are here
2. Creation of the BillingAccountContact model
3. Implementation of BillingAccountContact and sync with Zuora
4. Data backfill for existing BillingAccount objects

Problem

There's a BillingAccountContact class that's currently being called from BillingAccount#contact.

Proposal

We want to make the BillingAccountContact class name available for the new model. This issue tracks effort to rename the existing BillingAccountContact into something else.

Result

Passing tests.