Add crimes affecting women per capita to annotations

Addresses https://github.com/sc3/cook-convictions/issues/87
Bake convictions by age bucket JSON

Add the logic to create counts by age bucket to the Queryset
instead of having it in a snippet in the IPython notebook.

Add a management command to do the export.

Remove records for those < 18 from our analysis.

Addresses https://github.com/sc3/cook-convictions/issues/86