* 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
* Round time_on_page metric returned by 'pages' API
This rounds the `time_on_page` metric returned as part of the `pages`
API to the nearest second. While this shows no apparent change in the
web UI, it removes the decimal from the exported data in `pages.csv`.
* Tidy up export tests
* Round time_on_page in db query
* Add goal to CSV export tests
* Display CSV export spinner until download is ready
The mechanism used to make the page aware that the download is ready is:
- Client code sets a cookie and requests download.
- Server code handles download and removes cookie when complete.
- Client code polls every 1s to check the cookies, removing spinner when
the export cookie is removed.
