Added a base model class for future models to extend. Includes db.model,...

... primary key ID, date created, and date modified.
Break the date columns into a mixin:

class DateCreatedModifiedMixin(object):
    date_created = db.Column(db.DateTime, default=db.func.current_timestamp())
    date_modified = db.Column(db.DateTime, default=db.func.current_timestamp(), onupdate=db.func.current_timestamp())