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.
It would be nice to see some updated test for this change. It would also be helpful to have an updated docstring, now that this function actually returns a value.