Evaluating formulas on `/billable_events` is slow

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One month of /billable_events for a large organisation evaluates the pricing formulas about 30,000 times. This takes >3 seconds and contributes to that API endpoint being slow. There are several things that could be tried to improve this:

• Relatively few of the pricing plans actually need to be dynamic. Many of them only need to multiply the duration by a fixed per-second cost;
• #73 should allow evaluating these formulas only once, when creating billable_event_components in the background.