Use webpack to deliver newest ACE editor

With this commit, we change the delivery method for the ACE editor from manually copied files to using yarn and webpack. As a side-change, we also modify how the mode is selected through JavaScript instead of Ruby.

Through webpack, the `modePath`, `themePath`, and `workerPath` are automatically determined and working as expected.

Closes #250

app/assets/javascripts/markdown_ace_editor.js

@@ -1,8 +1,5 @@
 (function() {
     window.MarkdownEditor = function(selector) {
-        _.each(['modePath', 'themePath', 'workerPath'], function (attribute) {
-            ace.config.set(attribute, CodeOceanEditor.ACE_FILES_PATH);
-        }.bind(this));
         var editor = ace.edit($(selector).next()[0]);
         editor.on('change', function() {
             $(selector).val(editor.getValue());