Followup 2f2da72747
This commit moves topic view tracking from happening
every time a Topic is requested, which is susceptible
to inflating numbers of views from web crawlers, to
our request tracker middleware.
In this new location, topic views are only tracked when
the following headers are sent:
* HTTP_DISCOURSE_TRACK_VIEW - This is sent on every page navigation when
clicking around the ember app. We count these as browser page views
because we know it comes from the AJAX call in our app. The topic ID
is extracted from HTTP_DISCOURSE_TRACK_VIEW_TOPIC_ID
* HTTP_DISCOURSE_DEFERRED_TRACK_VIEW - Sent when MessageBus initializes
after first loading the page to count the initial page load view. The
topic ID is extracted from HTTP_DISCOURSE_DEFERRED_TRACK_VIEW.
This will bring topic views more in line with the change we
made to page views in the referenced commit and result in
more realistic topic view counts.
