admin-panel: refactor + standardize datetime handling

Started out as just handling latest changes to datetime handling, but ended up doing a lot of refactoring:
1. Used starlette admins own way to handle datetime presentation/conversion (TimezoneConfig)
2. Create a DivBaseModelView class with default settings for all model views to inherit from
3. Catch/include various missing fields that were not being shown in the admin panel.
4. Helper fns to append common fields to the list of fields to show in the admin panel.
5. Improve docstrings/help text/labels.

Author: RMCrean