Added some metrics to help us determine how 'fresh' our fronts are

For the 3 edition fronts (/uk, /us and /au) we will get the average time in seconds since the page was rendered.

For other pages we will get buckets, e.g. less than a minute, less than an hour and so on. 

The buckets are (slightly) arbitrary, but I want to get an idea of the distribution first, and then we can have a think about it.

Later on we can possibly set alerts based on this.