Change Web UI to use /master/redirect for leader redirection.

The leader address from master/state.json does not undergo the same
name lookup as the master/redirect end point. This becomes a problem
during redirect in the webui when internal IPs are used.

Changing location of the redirect to '/master/redirect' solves the
immediate problem.

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/14859