Update fco links to prison packs in Arrested Abroad calc
Update links in prisoner packs data file
Replace all old FCO links with new Gov.uk links
Links don't need to be external anymore
Don't presume the link is external for prisoner packs
Update tests following link changes
Remove lawyer link to Peru

We currently don't use these lawyer links and also this one is old and
404s, there is no redirect in place.
Add test for generating download link markdown
Merge pull request #299 from alphagov/removing-fco-links

Removing FCO Links: Arrested Abroad