Switch to Bootstrap 4

app/assets/javascripts/application.js

@@ -10,9 +10,10 @@
 // Read Sprockets README (https://github.com/sstephenson/sprockets#sprockets-directives) for details
 // about supported directives.
 //
-//= require jquery
-//= require jquery_ujs
 //= require turbolinks
+//= require jquery3
+//= require rails-ujs
+//= require popper
 //= require bootstrap-sprockets
 //= require codemirror/codemirror
 //= require codemirror/addon/display/placeholder

app/assets/javascripts/init.coffee

@@ -18,6 +18,6 @@ App.init = ->
 $(document).on "turbolinks:load", ->
   App.init()
 
-$(document).on "click", '#translationNav a[href="#preview"]', ->
+$(document).on "click", '#translationTab a[href="#preview"]', ->
   $('#preview .problem-content').html(cm.getValue())
   MathJax.Hub.Queue(["Typeset",MathJax.Hub]);