LearnPress Assignment Add-on 4.1.2

~ Refactor, optimize code.
~ Tweak all layout on the Frontend.
~ Tweak display on the Backend.

~ Compatible with addon Frontend Editor 4.0.2.
~ Remove "learn_press_get_user_item_id" function.
~ Check $user->get_course_data, $course_data->get_item.
~ Remove some files use for old Frontend Editor (lower 4.0.2).
~ Fixed: error not send mail when student submit answer.
~ Fixed: error display one line on the content of answer.

~ Replace: all LP_Global::course to learn_press_get_course()
~ Fixed : translated item assignment with Addon LP WPML.
~ Fix: upload limits.