Add visibility.js, wrapper on page visibility API
Use visibility.js to run timer updates

Visibility.js provides a thin wrapper over page visibility API, which
allows to detect if page is currently visible by user. This allows us to
stop live updates when it's not needed. This is especially easy in case
of timers, because Visibility.js provides setInterval replacement, which
runs given code only when page is visible.

A lot of :heart: for @tchack, who showed me visibility.js!