Era Time-left pie-chart / timer

[rossbulat]

We would like to let the user know how much time is left in the current era.

What would be nice is if we had a circular countdown styled the same way as the pie charts for stat boxes:

And on hover, show the remaining time left of the era:

One possible solution is to use this same chart js pie chart, and wrap it in a useEffect / setInterval loop that updates time left based on blocks.

We would put this timer alongside the Active Era stat on the Stake and Validator pages. It currently has no accompanying chart: