Fix for overlength errors when creating courses

The SIS batch import created by the Start a New Course form will fail silently if `long_name` is longer than 255 characters (= max length of corresponding field in the database). There have been two tickets related to this issue (#317466 and #318781).

This fix attempts to shorten any overlength cross-listed course names, and warns the user if it is unable to shorten the name enough.