Add config flag for disabling option to "Clear Schedule"

I.e. when editing the schedule

tailbone/views/shifts/schedule.py

@@ -121,6 +121,8 @@ class ScheduleView(TimeSheetView):
 
         context['form'] = form
         context['page_title'] = "Edit Schedule"
+        context['allow_clear'] = self.rattail_config.getbool('tailbone', 'schedule.allow_clear',
+                                                             default=True)
         return self.render_full(**context)
 
     def clear_schedule(self):