Add feature to display detailed user activity in Sitestats

It is useful for instructors to be able to see the activity of students in their course for various reasons. This is possible to some degree in certain tools like Tests & Quizzes, and also in a limited way using the Statistics tool.

This feature adds a User Activity page to the Statistics tool which provides a central place to see a chronological listing of all events the user has performed in the site (that Sitestats is aware of), and can provide details about the events on demand.

Please see the attached screenshot and video for how this feature works at Western. We've had it in production for over a year.

For an idea about performance, note that in the video the queries are being made against a table with 250 million rows, and the "show more" lookups are happening on demand.

NOTE: The "poll.vote" event is marked as "anonymous" for the reasons described in --SAK-28679-- (and there are other scenarios not described there that make it possible to deduce a voter's selections). However, as tracking this event may be valuable for users of the User Activity feature, this event is technically supported by User Activity. Institutions that wish to have this event display in User Activity can do so by modifying toolEventDef.xml to remove the anonymous attribute from "poll.vote".