Persist Spark UI link after kernel goes down

[jonathanindig]

Right now, when a kernel crashes the Kernel UI reverts to the "stopped kernel" view, which no longer shows the Spark UI link.

Since the UI link could possibly be a HistoryServer link in some deployments, it would be useful to have an option to persist the link (or even better, if we could automatically detect that the link remains valid after the kernel is shut down and persist it).