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

test/models/trend_test.rb

@@ -1,37 +0,0 @@
-require "test_helper"
-
-class TrendTest < ActiveSupport::TestCase
-  test "up" do
-    trend = Trend.new(current: 100, previous: 50)
-    assert_equal "up", trend.direction
-  end
-
-  test "down" do
-    trend = Trend.new(current: 50, previous: 100)
-    assert_equal "down", trend.direction
-  end
-
-  test "flat" do
-    trend = Trend.new(current: 100, previous: 100)
-    assert_equal "flat", trend.direction
-  end
-
-  test "infinitely up" do
-    trend1 = Trend.new(current: 100, previous: nil)
-    trend2 = Trend.new(current: 100, previous: 0)
-    assert_equal "up", trend1.direction
-    assert_equal "up", trend2.direction
-  end
-
-  test "infinitely down" do
-    trend1 = Trend.new(current: nil, previous: 100)
-    trend2 = Trend.new(current: 0, previous: 100)
-    assert_equal "down", trend1.direction
-    assert_equal "down", trend2.direction
-  end
-
-  test "empty" do
-    trend = Trend.new
-    assert_equal "flat", trend.direction
-  end
-end