Restrict `locale` to list of valid locales
Restrict `locale` to list of valid locales

It's currently possible to provide a bogus locale, which would cause
frontends to crash because they set the Rails locale to this string
(`I18n.locale`).

This list was generated by running this on government-frontend rails
console:

```
I18n.available_locales.map(&:to_s).sort
```

alphagov/govuk-content-schemas,alphagov/govuk-content-schemas