Add explainer for db-unchanged

helpers/db-unchanged.js

+// when data changes, realm gives us an object with updates
+// https://realm.io/docs/javascript/latest/#collection-notifications
+// but it sometimes gets fired even though there are no changes
+// - we want to check for that and see if all arrays are empty
 export default function (dbChanges) {
   return Object.values(dbChanges).every(changeArray => changeArray.length === 0)
 }
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