Add tasks for deduping stats announcements

This adds a couple of fabric tasks around the [de-duping whitehall stats
announcements](https://github.gds/pages/gds/opsmanual/2nd-line/applications/whitehall.html#de-duplicating-whitehall-statistics-announcements) process. It makes me nervous that we have a common task that involves us SSHing onto a production machine and manually running a specific command as a
specific user from a specific location. This just wraps a little bit of that up.

I'm undecided as to whether this is a good idea, there is a noise cost to adding fabric tasks. It feels like this should be a web interface to do this in the publishing tools rather than a fabric script.