Course start time displayed based on timezones

Currently, course start times are displayed solely in UTC time. This can be confusing for instructors,
since they might schedule the start time in their timezone only to another date advertised due to
the conversion to UTC.

To fix this discrepancy, I've edited the text that is returned when a course's start date is requested
to be displayed based on timezone settings. This is done by updating the strftime_localized function in
date_utils to also take in a timezone; thus, this function may now be used elsewhere when a date displayed
in an appropriate timezone is desired.

This is a fix for the issue logged here: https://openedx.atlassian.net/browse/LOC-61