Fix RecurringRule and OccurringRule model __str__ method

[saadmk11]

1. Django admin raises error on RecurringRule model admin page (change_list). This is caused by the __str__ method.

   Traceback:

   pythondotorg-web-1       |   File "/code/events/models.py", line 287, in __str__
   pythondotorg-web-1       |     return f'{self.event.title} every {timedelta_nice_repr(self.interval)} since {date(self.dt_start.strftime, strftime)}'
   pythondotorg-web-1       |   File "/code/events/utils.py", line 50, in timedelta_nice_repr
   pythondotorg-web-1       |     raise TypeError('First argument must be a timedelta.')
   pythondotorg-web-1       | TypeError: First argument must be a timedelta.

2. RecurringRule and OccurringRule model admin page (change_list) does not display the expected __str__.

   Expected: Djangocon Europe 2014 05/13/2014 midnight - 05/17/2014 midnight
   Actual: Djangocon Europe 2014 -