Add start/end time fields to CalendarEvent (#28) (#29)

## 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

schasite/admin.py

@@ -165,10 +165,23 @@ class CalendarEventAdmin(admin.ModelAdmin):
     list_display = [
         "event_name",
         "start_date",
+        "start_time",
         "end_date",
+        "end_time",
         "coordinator_email",
         "event_link_name",
     ]
+    fields = [
+        "event_name",
+        "start_date",
+        "start_time",
+        "end_date",
+        "end_time",
+        "location_name",
+        "coordinator_email",
+        "event_link_name",
+        "event_url",
+    ]
 
 
 class AddressModelAdmin(admin.ModelAdmin):