Admin interface should allow entering currency amounts as dollars/cents, rather than just a whole number of cents.

[zenhack]

Right now on the admin page, if you want to e.g. add a payment for $7.00, you have to enter "700." This is confusing. Currencies are stored as cents in the database, but we should be exposing that in the UI.

Currencies are displayed correctly, but the inconsistency almost makes it worse.