Speed up Whitehall mappings import by 50%

Until now, we have been loading the host for each old URL in the CSV from the
database each time. This manually caches the hosts (and semi-accidentally the sites too).

In development this changed the runtime from:
```
FILENAME=tmp/1396018394-whitehall_mappings.csv bundle exec rake   181.65s user 112.01s system 88% cpu 5:31.37 total
```

to:
```
FILENAME=tmp/1396018394-whitehall_mappings.csv bundle exec rake   95.10s user 54.83s system 86% cpu 2:53.45 total
```