Fix issues with GA

The event is submitted to Google asynchronously. When we do the form
submit, the context of the page switches and the event submission no
longer gets processed.

To overcome this, we push the trackEvent command to Google's Analytics
queue. Once that has been pushed to the queue, we push the command to
submit the form.
Workaround for GA async event tracking

Since pushing the event to GA's queue did not work, we use a small hack
to track events + submit the download form. We push the event to GA's
async queue and then submit the form 300 milliseconds later. This should
work (in theory) and should not result in a noticable user-facing change
at the same time.
Use Mixpanel for event tracking instead of GA
Fix Mixpanel event tracking call

Instead of using .track_forms - which didn't quite work - use .track().
Add the form submission in the callback. If the event tracking fails, we
have a 1 second timeout that we use as a fallback.

Event tracking is now working.
Track initial pageview