Fewer queries on stats/scales + Fix

Scales:
- Fix #chunks when used with std_chunk=True (there was one too many at
  the beginning)
- Scale.end gives the end of the last chunk (instead of its start)
  So scale.begin -> scale.end gives the full range of the scale.

`kfet_day` now returns an aware datetime.

ScaleMixin:
- new method `get_by_chunks` which use only one query and ranks
  elements according to the scale. Elements are returned by a generator
  for each scale chunk (and all chunks are returned as a generator too).

ArticlesStatSales and AccountStatOperations use this new method to
avoid issuing #scale_chunks queries.

ArticleStat:
- fixed on Chrome

kfet/static/kfet/js/statistic.js

@@ -61,7 +61,7 @@
                 var chart = charts[i];
 
                 // format the data
-                var chart_data =  is_time_chart ? handleTimeChart(chart.values) : dictToArray(chart.values, 1);
+                var chart_data =  is_time_chart ? handleTimeChart(chart.values) : dictToArray(chart.values, 0);
 
                 chart_datasets.push(
                     {
@@ -132,7 +132,7 @@
                     type: 'line',
                     options: chart_options,
                     data: {
-                        labels: (data.labels || []).slice(1),
+                        labels: data.labels || [],
                         datasets: chart_datasets,
                     }
                 };