We fetch the cost of gift badges and suggested one-time donation
amounts. For example, gift badges cost $5 and £4.
Previously, we parsed these as `UInt`s. This had two disadvantages:
1. There was no way to represent fractional amounts (like $1.23)
2. It was inconsistent with a lot of our code, which expects `Decimal`s
or `NSDecimalNumber`s
This changes these to use `Decimal` instead.
Tested this by:
- Adding some automated tests
- Making a one-time donation (with a non-default currency)
- Sending a gift badge (with a non-default currency)
- Checking donation receipts
We also had some hard-coded presets for one-time donations, which were
unused. I removed these.
e0ebfac315