Cache counts of event comments, talks and tracks

This avoids potentially expensive subselects when retrieving event information. I've added the following fields to the `events` table:

* `comment_count`
* `talk_count`
* `track_count`

There's some methods in the EventMapper to update these values.

Frisby tests check for the existence of the new properties.

I'll also adjust the existing joind.in site to update these values whenever relevant new entities are created.