LOS ANGELES (Reuters) - Grubhub Inc's first-quarter net income tumbled 78 percent after it ramped up spending to attract and retain restaurant partners and delivery customers. GrubHub, which is battling startups ranging from DoorDash and Uber Eats to Amazon.com Inc's Amazon Restaurants, has swept up more than a half-dozen companies since its April 2014 initial public offering and has been spending aggressively to expand its delivery network. Sales and marketing outlays were $78.5 million (60.9..