Fix core member count

Fix the core member count to actually work when generating a report for
multiple projects (such as all projects).

Change-Id: Ib846c5e5d42478505bd622c0f7adf105c300ed13
Show average reviews per day

Show average reviews per day across all reviewers as well as across the
core review teams.

Change-Id: I73cb6359d4f256c097c555b9b79d375042877cb7