This adds a new emergency mode that can be enabled by the owner of the system.
When in emergency mode, stakers or `StakeVault`s can leave the system immediately.
This also applies when there was a malicious upgrade and a call to
`emergencyModeEnabled()` panics.
To have this in a fully secure manner, we still have to add the counter
part of "leaving" the system. This will allow users that don't agree
with a (malicious) upgrade to get their funds out of the vaults
regardless.
Closes#66