rdevlin.cronin@, could you list all the other URLs that previously existed and that we still need to support in MD Extensions?

I searched through the existing extensions code and only found /configureCommands, so will move forward with just that unless someone finds otherwise.

The only other url-related support I can think of is using the query params with id=<extension_id> to highlight the extension (replaced with navigating to the extension detail page), which should already be handled.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3c45abea5a4e62a95c08331d5d88b4c8b1e8e3bb

commit 3c45abea5a4e62a95c08331d5d88b4c8b1e8e3bb
Author: Scott Chen <scottchen@chromium.org>
Date: Thu Oct 26 15:15:29 2017

MD Extensions: reroute legacy paths to new paths.

This CL adds code to reroute legacy paths to their equivalent MD Extensions
paths. It also moves route-related code in NavigationHelper's constructor
to a separate helper method, since it's growing to be quite a few lines.

Bug:  775698 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I473ff4c2e3fb4bedb814b9538ee2a46f11459b13
Reviewed-on: https://chromium-review.googlesource.com/724271
Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511822}
[modify] https://crrev.com/3c45abea5a4e62a95c08331d5d88b4c8b1e8e3bb/chrome/browser/resources/md_extensions/navigation_helper.js
[modify] https://crrev.com/3c45abea5a4e62a95c08331d5d88b4c8b1e8e3bb/chrome/test/data/webui/extensions/extension_navigation_helper_test.js