Add explicit grouping by filter option

What does this MR do and why?

Add explicit grouping by filter option

Closes #139 (closed)

Add help for new grouping option

This also moves the help content into a drawer, as it's a bit much for a popover.

Review/run this locally

1. Copy the pages in mr job URL (must have succeeded)
2. Run bin/review-mr.sh <job url>
3. If there are rules changes, check the compare findings job log reports the expected changes
4. If there are group changes, check the compare groups job log reports the expected changes

Closes #139 (closed)