Staticweb doesn't handle double-slashes in object names well

Specifically, it makes a link to "/" -- i.e., the server root -- which will almost certainly 404.

If we force the href at https://github.com/openstack/swift/blob/2.25.0/swift/common/middleware/staticweb.py#L326 to start with "./", it should fix it.

I could see someone trying to use this as a "feature" to hide some parts of the container -- but it wouldn't work very well: if a user manually adds the extra slash at the end of the URL, the listing works as expected. Which is to say, I think we can just fix the bug and not worry about upgrade concerns.