Script for cleaning up duplicate announcements

Some code that will make it easier to delete and redirect duplicate statistics announcements in future. It registers a redirect from a duplicated announcement to an authoritative one and destroys the
duplicate. The script can be run from the command line:

```
$  ./script/dedupe_stats_announcement dupe_slug authoritative_slug
```

The script also accepts a "-n" option to call the command as NOOP.

Story: https://www.agileplannerapp.com/boards/173808/cards/5804
it'd be clearer if this method was named: `duplicate_announcement_path`