Forum summary report: Create behat tests for dates filter, report download and links to export

The forum summary report requires behat tests for the dates filter functionality, as well as the links to forum export.

The tests should include:

1. Report with from date only.
2. Report with to date only.
3. Report with from and to date set.
4. Report with from date, to date, and groups filter set.
5. Only users with any activity have an export link.
6. Following an export link with from/to date and groups filter set and pressing the export button loads the export with the relevant filters applied.
7. Ability to download report.