In private emails in 2015 and 2016, President Donald Trump’s top immigration adviser touted a vilely racist novel that warns of a migrant invasion, promoted the ideas of white nationalist publications, and raged at retailers who stopped selling Confederate flags in the wake of the massacre of black churchgoers in Charleston, South Carolina. On Tuesday, the Southern Poverty Law Center published excerpts of emails Stephen Miller, the architect of Trump’s assaults on immigrants, sent to the right-wing outlet Breitbart.