Tracing statistics: simplify widget initialization

There is no dedicated view class needed for the statistics widget. A very simple
template is returned. Therefore, this commit employs Django's built-in
TemplateView directly. The URL to retrieve this widget has been simplified as
well: instead of /statisticswidget is now reachable with /stats.
Tracing statistics: split page refresh in sub methods

This improves in my eyes the readability of the code.
Tracing statistics: ensure statistics are loaded after layout

This commit moves the request of the actual statistics in the callback of the
layout loading, because we can't know how long it takes for the layout to be
available.