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

config/initializers/constants.rb

@@ -1 +1,7 @@
 EXCHANGE_RATE_ENABLED = ENV["OPEN_EXCHANGE_APP_ID"].present?
+
+BALANCE_SHEET_CLASSIFICATIONS = {
+  asset: "asset",
+  liability: "liability",
+  equity: "equity"
+}.freeze