Fires custom event on action page. Fixes #3308.

# Changes
 - Adds a [custom event](https://developers.google.com/analytics/devguides/collection/gajs/eventTrackerGuide) to the action page. See #3308 for context.

# Question
 - Is there a better place for this? This method seemed mostly concerned with preprocessing variables for theming, but it seemed to be the most appropriate place I could find.

For review: @aaronschachter