The base path is an absolute path, so it should always start with /.
Whenever it is used, it must also always be followed by at least one /,
so baking that into the value makes sense. Finally, we can now
deduplicate the / in case the base path is the root.
While this is one more thing to configure when running tablejohn as a
web server, the overhead should be negligible if I provide an example
server config. Security trumps convenience - this time.
