Track app detail page visits (Bug 965965)

This is a second try at [Bug 965965](https://bugzilla.mozilla.org/show_bug.cgi?id=965965). In https://github.com/mozilla/zamboni/pull/1745, an alternative was proposed which was to create a new separate API endpoint that would solely be responsible for tracking app detail page views. This pull request is an attempt at that.

This relates to: https://github.com/mozilla/fireplace/pull/368