added script to update mirage-www when (at least one) build status changes

This is the partner PR to https://github.com/mirage/mirage-www/pull/235

See header comment in update_mirage-www.sh (the sole file included in this PR) for a description of what it does.

The script works in my setup, but it could do with further streamlining of how it expects to find authentication credentials.

minor: should be `/usr/bin/env bash` if you really need bash (with a separate `set -ex` in another line if that's the case).  `/bin/bash` is not portable.