Set up a GitHub bot to automatically post links to new PRs into the #request-review or #test-datasets slack channels.

[SPPearce]

Currently we have no automated reminder for test-datasets, and nobody really visits on their own.
We could do with some way to increase the visibility of open PRs so they don't sit around ignored.

This could include:

• An automated github bot to ping the #request-review or #test-data channels.
• Automatically pinging people to review a PR when it is made (which doesn't seem to happen currently).