PARIS — With the end of France’s summer vacation season, the election campaign to replace Emmanuel Macron as president next May is bursting to life. An unusually large and broad field of candidates is forming to lead the European Union’s largest country, its second-largest economy and only nuclear-armed power, their appetites whetted by the fact that Macron is constitutionally barred from seeking a third consecutive presidential term himself.Some won’t get the support they will need from sponsors and their political camps to be on the starting line when the first round of voting is held on April 18. Still, a...