[SPARK-20042][WEB UI] Fix log page buttons for reverse proxy mode

with spark.ui.reverseProxy=true, full path URLs like /log will point to
the master web endpoint which is serving the worker UI as reverse proxy.
To access a REST endpoint in the worker in reverse proxy mode , the
leading /proxy/"target"/ part of the base URI must be retained.

Added logic to log-view.js to handle this, similar to executorspage.js

Patch was tested manually

Author: Oliver Köth <okoeth@de.ibm.com>

Closes #17370 from okoethibm/master.