support: Show annual revenue for active fixed price plans.

[laurynmm]

In the activity and support views, we want to see the annual revenue for fixed price plans. While on billing pages, we do not display this information as these plans are renegotiated annually.

Adds get_annual_recurring_revenue_for_support_data function to BillingSession class, so that we can get the fixed price plan data for these views without changing the logic for what is displayed on the billing pages.

Screenshots and screen captures:

Remote support view

Before	After

Remote activity view

Before	After

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Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

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[timabbott]

Doesn't this depend on the billing cycle? I think fixed-price with monthly billing may be a possible variation.

Also, what's the thinking around whether we should instead be changing get_customer_plan_renewal_amount?

[laurynmm]

Fixed price monthly billing is possible, but the amount in the fixed_plan field of the model is the annual amount in cents. That would probably be useful documentation to add. But my previous changes were incorrect, in that the non-fixed price plans were returning an amount based on the billing schedule (monthly or annually).

I've updated the new helper function to always return the annual revenue amount.

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Re: why add a new function instead of updating get_customer_plan_renewal_amount is that for user-facing billing pages, we want to show this:

        if plan.fixed_price is not None:
            if plan.end_date == self.get_next_billing_cycle(plan):
                return 0

And for activity/support pages, we want to show the annual fixed price amount.

[timabbott]

OK, looks good, marked to merge once CI passes.

I feel like I'm never going to get to doing a pass of comment documentation on corporate/models.py; maybe you should do a pass on documenting the fields there, and I can do a follow-up for anything unclear after reviewing that?