Remove support for datetime objects in attributes and extras

[szoupanos]

Currently when the attributes and extras are stored, there is a datetime_to_isoformat conversion and the opposite action (isoformat_to_datetime) is performed when a JSON field is read.

If we remove datetime serialization (issue #864) the implementation of Django JSONB should be changed and adapted as the SQLA JSONB.

We should see in both issues what we do with DbSetting that also contains dates and is stored in JSONB.