IATI Hub: support added for 'Target Groups' description

Small update to IATI activity file exports — the Beneficiary Description field on the Activity Overview tab will now be published as the IATI "Target Groups" description. This is in addition to the preexisting support for Context and Objectives fields publishing as "General" and "Objectives" descriptions respectively.

Publishers are encouraged to publish all three IATI description types, but at least one is required to generate an export from DevResults.