* Fix custom property total conversions value not displayed
The custom property conversion metrics are not consistent with the other
metrics resulting in the total conversions not being displayed in the
dashboard. This fixes that.
* Export custom props of current goal when filtering dashboard for goal
This makes the CSV export also output a `prop_breakdown.csv` file which,
for the currently filtered goal, contains the conversion data for each
of its configured properties.
* Add test for goal-filtered CSV export
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