Currently, when a page billing subscription is updated, we call `reset_arrears_if_needed`, which set the arrears back to 0 if there is only ONE page on the org (i.e., the page getting the subscription update). However, this can cause problems for customers who have trial pages. Consider the following:

Steps to reproduce:

1. Customer has a Stripe page A and a Trial page B
2. Customer exceeds arrears on Stripe page A, resulting in a banner that tells them to pay up
3. Customer talks to support and switches to an invoiced plan on page A
4. The arrears are not reset because the customer has a Trial page B
5. The customer still gets the billing banner even though they have updated their billing

Expected behavior:

• reset_arrears_if_needed should set the arrears back to 0 for this customer. Here's the logic we should use:
  DON'T reset arrears on the org if they have any other paid pages (other than the page being updated). Otherwise, reset the arrears. This means that orgs with only trial/comped pages should always get their arrears reset when updating their primary page subscription.

Alternatively, we could just always reset the arrears on a subscription update. If a customer is changing their billing type, what are the chances they still aren't paying for another page and need to see the banner? Chances are, they're going to be upset they're still seeing a banner after updating their subscription.