Even though the AWS S3 documentation states that we should do it,
neither Uppy Companion nor tusd are doing this and nobody complained, so
apparently there is no need. If it was really that important, I would
expect aws-sdk-s3 to have a method for "safe aborting".

Since it's good to take every opportunity to make code simpler, we
remove the code that retries aborting the multipart upload.

This was changed in Uppy 1.0

Makes for a nicer `rake routes` command output in Rails apps.

Unlike some other web frameworks like Roda, Rails adds a trailing slash
to PATH_INFO when calling mounted apps on root requests. This means that
for `POST /s3/multipart`, the Uppy::S3Multipart::App will receive

  SCRIPT_NAME: "/s3/multipart"
  PATH_INFO: "/"

instead of

  SCRIPT_NAME: "/s3/multipart"
  PATH_INFO: ""

So we update the app to handle that trailing slash.

:create_multipart_upload and :object_url are probably the only options
that will be used, so it makes sense to show them in the examples.

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