Hid links to dead slaves to prevent dead links.

If a slave is gone, the framework page will print "Slave gone" in place
of the links to the slave's UI. The links would previously fail in
redirection and return the user to the Mesos root page, which is
unpleasant.

Fixes: https://issues.apache.org/jira/browse/MESOS-650

Review: http://reviews.apache.org/r/14074