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.
Merge pull request #80 from alphagov/add-whitehall-de-dupe

Add tasks for deduping stats announcements