Implement UrlHelper middleware

[weierophinney]

As noted in a comment on a zend-expressive issue, the current approach to registering the UrlHelper is not feasible, due to the following issues:

• In cases where the UrlHelper is pulled during creation of the TemplateRendererInterface instance, and a TemplatedErrorHandler (or derivative, such as the whoops error handler) is also used, a circular dependency occurs. This is due to the UrlHelperFactory also pulling the Application service (as the TemplatedErrorHandler instance is pulled inside the ApplicationFactory).
• In cases where the UrlHelper creation is delayed until first use (e.g., in zend-expressive-zendviewrenderer's url helper factory), the helper is created after route result observers are updated, which means the helper never receives the route result.

This patch does the following:

• Creates UrlHelperMiddleware, which composes a UrlHelper and a RouteResultSubjectInterface instance; when invoked, it attaches the helper to the subject as an observer.
• Creates UrlHelperMiddlewareFactory, which creates a UrlHelperMiddleware instance, using the UrlHelper service, and either the RouteResultSubjectInterface or Application service instance.
• Modifies the UrlHelperFactory to no longer pull the Application instance and attach the helper to it.

The plan is to update zend-expressive-skeleton to auto-register the UrlHelperMiddleware as pipeline middleware, just as we have done with the ServerUrlMiddleware.