Add cookie size monitoring New Relic metrics.

This adds middleware that will create custom parameter metrics in
New Relic to track the size of all the cookies being received for
our domain. The custom fields are "cookies_total_size" and a
separate named parameter for every cookie size, e.g.
"cookies.csrftoken.size".

This is intended to help us track cookie growth and better diagnose
issues where users lose their sessions. It is toggled by the
'request_utils.capture_cookie_sizes' Waffle Flag.