[keyserver] Add num revisions reviewed to monthly Phabricator leaderboard

Summary:
This adds a query for the number of revisions reviewed. It's there for both the monthly stats and the yearly stats.

Linear task: ENG-6306

Depends on D10477

Test Plan:

1. I created a Phabricator leaderboard thread in my local environment
2. I changed phabLeaderboardChannel with the value of that thread ID
3. I patched postLeaderboard to test the behavior on January 2024
4. yarn script dist/scripts/phab-leaderboard.js

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10478