Returns an array of sites objects, each of which contains an array of purchases.

@param {array} purchases An array of purchase objects.
@param {array} sites An array of site objects
@return {array} An array of sites with purchases attached.
function getPurchasesBySite( purchases, sites ) {
	return purchases
		.reduce( ( result, currentValue ) => {
			const site = find( result, { id: currentValue.siteId } );
			if ( site ) {
				site.purchases = site.purchases.concat( currentValue );
			} else {
				const siteObject = find( sites, { ID: currentValue.siteId } );

				result = result.concat( {
					id: currentValue.siteId,
					name: currentValue.siteName,
					/* if the purchase is attached to a deleted site,
					 * there will be no site with this ID in `sites`, so
					 * we fall back on the domain. */
					slug: siteObject ? siteObject.slug : currentValue.domain,
					isDomainOnly: siteObject ? siteObject.options.is_domain_only : false,
					title: currentValue.siteName || currentValue.domain || '',
					purchases: [ currentValue ],
					domain: siteObject ? siteObject.domain : currentValue.domain,
				} );
			}

			return result;
		}, [] )
		.sort( ( a, b ) => ( a.title.toLowerCase() > b.title.toLowerCase() ? 1 : -1 ) );
}