Have the various dates and datetimes be native dates and datimes.

[fgregg]

We have to do some things at the edge to deal with the fact they they are not, and it's a fact that we always have to remember. I imagine that the orginal developers wanted the flexibility of string format, but we have never needed it.

Example:

1. https://github.com/datamade/django-councilmatic/blob/b550d380365a1e525f2f721cae5a480a4ef59da5/councilmatic_core/models.py#L313-L322
2. https://github.com/datamade/django-councilmatic/blob/b550d380365a1e525f2f721cae5a480a4ef59da5/councilmatic_core/models.py#L714-L716
3. https://github.com/datamade/django-councilmatic/blob/b550d380365a1e525f2f721cae5a480a4ef59da5/councilmatic_core/models.py#L185
4. https://github.com/datamade/django-councilmatic/blob/b550d380365a1e525f2f721cae5a480a4ef59da5/councilmatic_core/models.py#L364

[fgregg]

relates to #150