Add command-line tutorial (PreGameController) and link it from the lobby

Adds a black-terminal tutorial page at /game/pregame/{session} that walks
players through help, pwd, cd, ls, cat, rm and sudo before the real game
starts. Links to it are shown on both the lobby (waiting for players) and
ready-up screens so players can practice, repeatedly, before starting.

webpack.config.js

@@ -14,6 +14,7 @@ Encore
   // main entry for your app
   .addEntry('app', './assets/app.js')
   .addEntry('game1', './assets/game1.js')
+  .addEntry('pregame', './assets/pregame.js')
   // split entry chunks for better caching
   .splitEntryChunks()
   // will require an extra script tag for runtime.js