Last October, Shopify Inc. CEO Tobi Lutke said his company’s goal was to “arm the rebels” against the Amazon.com Inc. empire. Since then, the mantra has become a rallying cry for Shopify’s employees and the merchant customers that use its e-commerce store software. And now, the business turmoil sparked by the Covid-19 pandemic is creating an opportunity for the online seller rivals to gain some valuable ground over their giant competitor.