[wpinet] Serve index HTML file from WebServer if available

[jwbonner]

It's currently a bit awkward to use the WebServer class for a web page that is intended to be opened in a browser (such as a dashboard), because the index page is always a directory listing rather than an HTML file. The solution here is just to replace the directory listing with the contents of index.html if such a file exists (I think this is fairly standard behavior for statically serving files). Note that the behavior when adding format=json is unchanged, which allows dashboards (e.g. Elastic) to get a directory listing even if index.html exists.