Constrain invalid slugs in the routes.

This will prevent blowing up when given invalid UTF-8 etc.
updates after factcheck and germany changes
minor copy changes to both calculators
Merge pull request #378 from alphagov/redundancy-calculators-update

minor copy changes to both redundancy calculators
Merge pull request #377 from alphagov/married-abroad-v2-may-20-updates

updates after factcheck and germany changes
Merge pull request #376 from alphagov/handle_invalid_utf8_slugs

Constrain invalid slugs in the routes.