Boohoo’s shares stumbled by more than 19 percent Monday morning—the most it has fallen on the London Stock Exchange since August—after the ultra-fast-fashion purveyor confirmed it’s looking for a new auditor following PricewaterhouseCoopers’s (PwC) decision to step down from the role at the end of its contract. PwC, which served as the PrettyLittleThing and Nasty…