* Rename `project_activity_summary` to `top_users_activity_summary`
to make clearer the distinction between it and the per-user summary.
* Rename `user_activity_summary` to `user_activity_timeline`, since
its output is structured a bit differently than the courser-grained
"summary" returned by the top-user query.
* Rename `ActivityDuration` -> `ActivityPeriod`