Things have looked grim for Austin’s upstart ridesharing companies since the return late last month of giants Uber Technologies Inc. and Lyft Inc. But that won’t stop Fasten Inc. from throwing a party to celebrate a redesigned mobile app. The company’s efforts to court riders and drivers over the past few weeks offer a telling look at how one small business aims to compete on equal footing with a pair of unicorns valued at more than $1 billion each.