MDL-35373 mod_assign Add form elements for assignsubmission_onlinetext word limit

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
MDL-35373 mod_assign Add functionality to enforce onlinetext word limit

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
MDL-35373 mod_assign Enable 'wordcount' plugin for TinyMCE editor

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>