[Feature] Add new notification: Livepeer participation rate passes 50%

[cristianmalfesi]

We can inform to the user when Livepeer participation rate passes 50% or falls below it.

Check these links:

https://forum.livepeer.org/t/what-happens-when-the-50-participation-target-is-reached/771
https://forum.livepeer.org/t/livepeer-participation-rate-passes-50/853