FCR UX tweaks

[michaelsproul]

Description

1. Create a new concept for reverts or "unconfirmations" which are the scary events that downstream users will want to track: a confirmed root that becomes unconfirmed 🙀.
2. Rename the current "revert" metrics to "restart" so they don't get confused with (1)

[dapplion]

Standarized metric names that address the naming issues https://github.com/ethereum/beacon-metrics/blob/master/metrics.md#fast-confirmation

[rahulbarmann]

I'd like to take this up!