Use mousedown instead of click so we also catch mouse wheel

Summary:
Reported by a reviewer, they don't see the alert when
mouse-wheel clicking.
According to the internet, mouseup should be triggered by
a mouse-wheel click, so I changed click to mousedown.

Test Plan:
- Put a link in a webpage, click with mouse, see alert
- Middle click with scroll wheel, see alert
  (I couldn't actually get my mouse-with-scroll-wheel hooked up,
   so I will have to check with a reviewer)

Reviewers: brian

Reviewed By: brian

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14501