Add basic summaries to groups in the overview page

When tasks in overview page are grouped, it would be useful to add a few key summaries to the currently empty row of the group. I’d suggest picking just the most crucial one, however eventually we could create a more robust system for this.

For starters just seeing this would help.
- number of tasks in the group
- number for each type
- number for each priority
- number of unassigned tasks