* 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.
