Extend the script added by commit 9bf97363b0 (Utilities/Release: Replace
upload step with a "push" script, 2019-05-24, v3.15.0-rc1~56^2) to
create the destination subdirectory for the CMake version being pushed.
Replace the `upload_release.cmake` script with a `push.bash` script
that is more configurable from the command line and that does not
hard-code any destinations. Instead of using `scp` to access
`cmake.org` directly, push the files atomically to a staging
directory from which another process will actually upload them.