Network Analytics Page for WP Multisite

[dannyvankooten]

Quoting @beckej13820 in #172:

I wish there was a network analytics page that could give some summary statistics on the entire network of WordPress Sites.

I'd like to be able to:

See overall traffic on all sites
Instead of seeing which pages have the most traffic, see which subsites have the most traffic
Be able to click the links in the report and go directly to a single sites analytics report

[beckej13820]

Thanks for preserving this as you move to a different system!