ga: convert to "tag manager"

There have been two generations of GA since we first implemented GA on
the website. The newest version allows us to get deeper insights while
collecting less personal data. This version is also faster and loads
async which should help load the &os; website on slower networks.

As a reminder: to opt out do ANY of the following:

• block google.com via an adblocker
• set (abuse) the "do not track" header

In addition we've selected to disable many features in the console that
would result in collecting more data.

With Hat: webstats@