Previously, WelcomePageHandlerMapping would forward to index.html.
This assumed that the static path pattern was always /**. If it had
been customised to, for example, /foo/**, then the forward would still
be to index.html and a 404 would result as the page is actually
available at /foo/index.html.

At first glance, it would appear that the forward should be made to
foo/index.html. However, as it's a forward rather than a redirect,
any relative URLs in the index.html page would then be resolved using
/ whereas they should be resolved using /foo/. This could be addressed
by using a redirect rather than a forward, but we don't want to do
that as it's more invasive and would require a roundtrip back to the
client. Instead, this commit simply stops performing the forward when
the static path pattern is not /**.

Closes gh-8788