This is a first pass at #11. A few notes on my implementation:

• The issuingEstimateURL and issuingEstimateAuth properties are added on to the IssuerResponse object instead of the IssuerRecord. This is purely because IssuerRecord is backed up by the on-device SQL store, and I wasn't entirely sure I wanted to worry about testing a SQL migration in an upgrade. (It should be fine, since I'm just adding 2 more columns, but still...)
• The outcome of that decision is that those values aren't stored on device. They don't persist. So a new request to the issuer is made each time you navigate to the Issuer screen. We'd have to make a network call anyway to get the estimate data, so this doesn't really add much load.
• It does make this bit of code more complicated. The IssuerResponse has the auth estimate URL, but only the IssuerRecord has the public address submitted to this issuer. So we have to get both and combine them before we can ask for estimates.