Allow custom formatting of due date strings.

STUD-724

Test course for allowing custom formatting of due date strings.

STUD-724

Try making the name of the course match the folder name.

More cleanup.

More cleanup.

updates.
Hide show_timezone from Advanced Settings and migrate courses with show_timezone=False to using due_date_display_format.
pylint/pep8
Style cleanup.
Unused import.
Merge pull request #916 from edx/christina/due-date-format

Allow custom formatting of due date strings.