Expose all WebServerSettings to application.properties and add external files path

This PR exposes all WebServerSettings to the `application.properties` file.  It also slightly revises the serving of external files.  External files are now served on a subpath of the context instead of co-mingling Pippo & external files directly on the context path. (i.e.  serve external files from `/context/ext` not `/context`).  The external files path is configurable via the API or `application.properties`.

Serving from multiple sources on the same exact path can create problems for other `WebServer` implementations.