GSF.Web: Add an option for web applications to redirect all 404 Not Found to a single page

[StephenCWills]

Adds an option to IWebPageControllerBuilder to redirect all responses that would return 404 Not Found to the Default Web Page instead.

This is done by registering a branching middleware before the WebPageController middleware on the main branch of the Owin pipeline. The branching middleware invokes the WebPageController middleware on the main branch, and if the response from the main branch is 404 Not Found, then it subsequently invokes a second WebPageController on a separate branch of the pipeline. The second WebPageController uses the same WebServer instance as the first, but has a slightly modified route template to force all resources to resolve to the default web page.

Since the second WebPageController is only invoked when the first responds with 404 Not Found, this will not mask other errors such as 500 Internal Server Error. And since the WebPageController maps all possible routes, this should only redirect requests that fail to locate resources on the following lines of code:

Embedded/static resource: https://github.com/GridProtectionAlliance/gsf/blob/6720c394fbc7da0e45a9df84e7922cf88ac37a35/Source/Libraries/GSF.Web/Hosting/WebServer.cs#L225

Cached resource: https://github.com/GridProtectionAlliance/gsf/blob/6720c394fbc7da0e45a9df84e7922cf88ac37a35/Source/Libraries/GSF.Web/Hosting/WebServer.cs#L263

However, it may be worth noting that if a custom route is added via IWebPageControllerBuilder.UseCustomRoutes(), and that route attempts to explicitly respond with 404 Not Found, this new setting would redirect it.

[clackner-gpa]

Added @ritchiecarroll for visibility. If no one has any comments I will merge tomorrow.