Change datetime fields to date fields in BagInfoMetadata model.

[McDaPick]

This PR is for Issue #506

After updating the date_start and date_end fields in the BagInfoMetadata model from

date_start = models.DateTimeField()
date_end = models.DateTimeField()

to

date_start = models.DateField()
date_end = models.DateField()

and running a migration, the date was being saved as one day earlier than what was in the bag-info file.

Transfer's Bag Info:

External Identifier: records-2017-12-11T20:09:48.238148
BagIt-Profile-Identifier: http://localhost:8000/api/bagit_profiles/2/
Bagging-Date: 2017-12-11T20:09:48.238148
Date-End: 2005-06-22
Date-Start: 2003-05-14

The Transfer's data through the Django shell after being imported:

>>> from bag_transfer.models import BagInfoMetadata
>>> x = BagInfoMetadata.objects.get(external_identifier="records-2017-12-11T20:09:48.238148")
>>> print(x.date_end)
2005-06-21
>>> print(x.date_start)
2003-05-13

Inside of the Transfer's detail view:

After reading @helrond comment about timezone/daylight, I started to take a look around and saw that config.py has the time zone set as America/New_York on line 15.

DJANGO_TIME_ZONE = "America/New_York"  # TZ database name for local timezone (string)

Updating this value to UTC allowed for the date_field to save correctly.

DJANGO_TIME_ZONE = "UTC"  # TZ database name for local timezone (string)

Transfer's Bag Info:

External Identifier: records-2017-12-11T20:09:48.238148
BagIt-Profile-Identifier: http://localhost:8000/api/bagit_profiles/2/
Bagging-Date: 2017-12-11T20:09:48.238148
Date-End: 2005-06-22
Date-Start: 2003-05-14

The Transfer's data through the Django shell after DJANGO_TIME_ZONE was updated:

>>> from bag_transfer.models import BagInfoMetadata
>>> x = BagInfoMetadata.objects.get(external_identifier="records-2017-12-11T20:09:48.238148")
>>> print(x.date_end)
2005-06-22
>>> print(x.date_start)
2003-05-14

Inside of the Transfer's detail view:

Side Note:

This update to the DJANGO_TIME_ZONE also seems to fix the bagging_date time being fives hours off of its value in bag-data.