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/application.js

@@ -19,9 +19,5 @@
 //= require flash
 //= require color_mode_picker
 //
-// vendor/assets
-//= require ace/ace
-//= require ace/ext-language_tools
-//
 // app/assets
 //= require_tree .