Change tutorial plugins into submodules

[AnnaTomanek]

The abbr, simplebox and timestamp plugins from here: https://github.com/ckeditor/ckeditor-sdk/tree/master/samples/assets/plugins should be loaded as submodules from: https://github.com/ckeditor/ckeditor-docs-samples

The following should be replaced:

• abbr is tutorial-abbr-acf
• simplebox is tutorial-simplebox-2
• timestamp is tutorial-timestamp