deploy: show full URL to the deployed index.html

Before, we would show the user that we had deployed into

    server/v1/account/container/path/

This relied on the staticweb middleware being installed and a suitable
web index being configured. We now show

    server/v1/account/container/path/index.html

instead when an index.html file is being deployed. If no index.html
file is uploaded (zapps with a non-standard UI), we show the same URL
as before and the users will have to sort things out by themselves.

We also show the URL unconditionally. Before, it was hidden as an
info-level log message.
I've updated the docstring!