Separate stats collection and presentation logic

[lukasgraf]

The logic for collecting stats and preparing them for presentation should be separated more cleanly, so we can introduce a bin/dump-content-stats command at a later point that dumps lines of JSON stats to a logfile like var/log/content-stats.log.

Currently all the logic lives in browser/content_stats.py

Ideally this would be split up into:

• A utility (not necessarily a zope.component utilty, just an encapsulated class) that knows how to collect stats, returning a Python data structure (nested dicts)
• A browser view that post-processes the output from that utility and renders it as JSON to be used on the frontend
• Templates that use C3 JS in such a way that it just loads the data from that JSON endpoint using data-url instead of injecting the stats data into data- attributes using server-side templates