Ipp middle band

[acrymble]

Closes #1988 - Middle IPP band

Adds a £400 rate for middle income countries to the EN/ES/FR pages.

The PT pages need a total redoing so I will update them all at once when we've had a chance.

I need my language checked on FR and ES:

• Paises de alto ingreso | Paises de ingreso medio | países AOD
• Pays à revenu supérieur | Pays à revenu intermédiaire | Tarifs échelonnés pour les pays éligibles à l'APD

Checklist

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[spapastamkou]

The numbers above thousand need to be written without comma and adding a space instead: 1,200 => 1 200. I will make the changes.

[spapastamkou]

I followed the official terms for categorizing countries according to income as per this list of the OECD. Please check if you are happy with the form @acrymble, I may have changed the link placement.