Refactor Django models to more efficiently use ODM2

[aufdenkampe]

• Migrate as much as possible from using the Django-managed dataloader database to ODM2 to eliminate duplicate data and data structures.
• Leverage SQLAchemy and Pandas libraries to simplify application code, improve maintainability, and increase performance.

This big task is a prerequisite for the 2021 AWS Imagine grant tasks and most of the 2022 Priority 1 features in our Monitor My Watershed Strategy, including requested features by the Watershed Institute.