Allow custom formatting of due date strings.

STUD-724

Allow specification of a format for due dates in LMS. See http://docs.python.org/2/library/datetime.html#strftime-strptime-behavior for syntax of format string.

For context, the issue here is that last fall we started showing the timezone on due dates. This bothered some course authors because they were using grace periods to "mask" what the timezone is. In response to their feedbac, we added a setting to hide the timezone ("show_timezone"). However, come this semester, it turns out that what they REALLY want is to hide the time part of the date completely. I decided to solve this by allowing them to completely control the formatting of the due date (which is how I wish I had done it in the past....).

Set the due date format in Studio, Advanced Settings, under the key "due_date_display_format".

Includes unit tests for the 2 places in LMS code where the due date is displayed (left-hand accordion, progress page).

Also removed a usage of django.utils.translation in common, as common should not depend on django. Currently we have no i18n process for things in common.