------
- handle a number of regressions
- consolidate model managers and managers on model mixins
to make it easier to predict the _default_manager. Users
should add unitests to confirm concrete model classes
have the expected _default_manager (all with model mixins)
- floor sec/microsec on datetimes when comparing in form
and form-validator validation (edc-appointment
edc-visit-schedule, edc-visit-tracking, edc-crf)
- remove `SiteModelMixin` from default on/off schedule
model mixins. Add this mixin manually to the concrete
class.