Zurich - allow external addresses to be flagged as deleted

The schema migrations need to be run before this will work correctly:

```
$ eval `./setenv.pl` && perl bin/update-schema --commit
```

Still need to:

- [x] Write a test to ensure deleted bodies can't have problems assigned to them.

Closes mysociety/FixMyStreet-Commercial#402
Closes mysociety/FixMyStreet-Commercial#261