On Tuesday evening, the dark recesses of the Internet lit up with talk of politics. “Tampa rally, live coverage,” wrote “Dan,” posting a link to President Trump’s Tampa speech in a thread on 8chan, an anonymous image board also known as Infinitechan or Infinitychan, which might be best described as the unglued twin of better-known 4chan, a message board already untethered from reality. The thread invited “requests to Q,” an anonymous user claiming to be a government agent with top security clearance, waging war against the so-called deep state in service to the 45th president. “Q” feeds disciples, or “bakers,”...