Add grouping by matched text

What does this MR do and why?

Refactor treemap utilities

This extracts reusable functions from some of the larger functions. This will be useful in later commits.

Extract matched text function

Again, this will be useful in later commits.

Add grouping by matched text

Closes #136 (closed).

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 #136 (closed)