This allows defining other /s3/* routes without potential clashes when
Uppy::S3Multipart::App isn't mounted last. One example of such route is
"/s3/params", which is called by the AWS S3 Uppy plugin.

We still keep backwards compatibility for users that currently have it
mounted on "/s3".

It's shorter, more intention revealing, and works across all browsers.

That should always work, regardless of whether the app is running
locally or remotely, so users can just copy-paste this.

We also switch to the raw XML format, as I think it's easy enough to
make the update through the AWS S3 Console after all.

Closes #1