Add the ability to "rollup" values in a time series (#554)

* Clean up time series models

* Add value group rollup class for summarizing hierarchical data

* Integrate new classes

* Update UI to use new patterns

* Update D3 charts to expect new data format

* Clean up account model

* More cleanup

* Money improvements

* Use new money fields

* Remove invalid fixture data to avoid orphaned accountables

* Update time series to work better with collections

* Fix tests and UI bugs

lib/money.rb

@@ -42,6 +42,10 @@ class Money
         @currency.default_format.gsub("%n", formatted_amount).gsub("%u", @currency.symbol)
     end
 
+    def to_json(*_args)
+        { amount: @amount, currency: @currency.iso_code }.to_json
+    end
+
     def <=>(other)
         raise TypeError, "Money can only be compared with other Money objects except for 0" unless other.is_a?(Money) || other.eql?(0)
         return @amount <=> other if other.is_a?(Numeric)