Handle missing exchange rate provider, allow fallback for missing rates (#955)

* Clean up exchange rate logic

* Remove stale method

db/migrate/20240706151026_rename_rate_fields.rb

@@ -0,0 +1,6 @@
+class RenameRateFields < ActiveRecord::Migration[7.2]
+  def change
+    rename_column :exchange_rates, :base_currency, :from_currency
+    rename_column :exchange_rates, :converted_currency, :to_currency
+  end
+end