Add a welcome page to the static resources

This is largely a precursor for the removal of the bundled webclient. The idea
is to present a page at / which reassures people that something is working, and
to give them some links for next steps.

The welcome page lives at `/_matrix/static/`, so is enabled alongside the other
`static` resources (which, in practice, means the client API is enabled). We'll
redirect to it from `/` if we have nothing better to display there.

It would be nice to have a way to disable it (in the same way that you might
disable the nginx welcome page), but I can't really think of a good way to do
that without a load of ickiness.

It's based on the work done by @krombel for #2601.

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