Enable withdrawals

[AgeManning]

We have built a new endpoint that allows a single POST request to voluntary exit validators. See here: https://github.com/sigp/lighthouse/issues/4117

This will enable the UI to very easly exit validators.

We should display a big warning sign, along the lines of "ARE YOU SURE YOU WANT TO EXIT" as well as like a type "Exit my Validator" before the UI does it. This is a non-reversible, permanent change to the validator.

We may even want to consider potentially having some time-lock or password such that if a user leaves their computer in a public space, someone can't come along and just exit their validators.

Maybe people could enter an optional password, that they need to re-enter after 10 mins of being idle or something before enabling destructive actions like withdrawing a validator or converting BLSToExecution changes.