## Summary
Closes#28.
- Add optional `start_time` / `end_time` (`TimeField`) on `CalendarEvent`
- Expose times in Django admin list + form
- Replace hardcoded calendar template times with `time_range_display`
- Add migration `0013` and unit coverage for time formatting
## Test plan
- [ ] `python manage.py migrate`
- [ ] `python manage.py test schasite.tests.CalendarEventTests`
- [ ] In admin, edit an event with start/end times and confirm calendar page shows them
- [ ] Confirm events without times hide the Time rowReviewed-on: #29
