Use chartkick globally

* Clean up legacy charting imports
* Update existing donut and line charts
* Create helpers for parsing out existing chart data
* Move chart partials to a more common path

app/views/shared/_big_statistic.haml

@@ -1,13 +0,0 @@
-- reverse ||= false
-- variant = variant_from_delta(delta, reverse: reverse)
-.dashbox.text-center
-  %span.fa
-    = fa_icon(icon)
-    = value
-  %p.text-nowrap
-    %small
-      = subtitle.pluralize(value)
-      %span.label{ class: "label-#{variant}" }
-        = delta
-        since you last logged in
-