Django-admin-charts

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* cleanups and refactoring
* faster queries
* add Django 3.0 support
* invalidate cache on models save
* dropped support of Python 3.5 (in which cache invalidation does not work)
* add AvgCountPerInstance operation type
* allow to set &debug=True GET parameter in chart-data view for easier debugging
* move distinct to separate field

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* fix js cache mismatches

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* add x_axis_format as DashboardStats field
* add interactive guideline to StackedAreaChart

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* fix problem with date as Date field
* report errors as javascript alerts

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* added support to display dynamic criteria as multiple series <Petr Dlouhý>
* chart type switcher was added <Petr Dlouhý>
* default values for charts switches can be configured in DashboardStatsAdmin <Petr Dlouhý>
* fix for USE_TZ=False <Petr Dlouhý>
* fix switches action that was not working in some cases <Petr Dlouhý>
* dynamic criteria values are automatically generated if dynamic criteria mapping not filled in (in some cases) <Petr Dlouhý>
* dynamic criteria JSON can now contain filter value <Petr Dlouhý>
* support for Django<2.0 and Python<3.5 was dropped <Petr Dlouhý>

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* removed remaining forced dependency on django-admin-tools <Petr Dlouhý>