This change converts the SidebarView to use react internally. This is

mostly a very straightforward change.
The only unusual part of this change is the shim between react-router

and Backbone.Router. For now, we're using react-router's <NavLink>

components, but intercepting click events and redirecting them back to

the old window.application.go() function. This triggers the existing

routes properly. We also have a handler for when the route changes in

Backbone to mirror that back into react-router's history object. Once

the other views have been converted (or at least wrapped), the

<Router> component can be moved into the application view, and we can

get rid of the special route handling and just rely on react for all of

that.

Verified that the sidebar items loaded correctly for a variety of

  scenarios.
Checked that the correct item was highlighted based on the route.
Checked that item highlighting updated correctly even when routes were

  triggered from elsewhere in the UI (such as the "All Users" list).