migrate data pivot to visual

[shapiromatron]

The DataPivot was a separate visualization type that even had a multi-table inheritance pattern, but in reality it is just a specific visualization type. This PR migrates all data pivots to be a specific visualization type, instead of a standalone model.

The main reason for the migration was delete a bunch of redundant code, and simplify cases where we want to show visualizations and data pivots on the same page, which would require 5+ queries instead of just a single query.

Migrate from this:

summary <django app>
├── Visual
└── DataPivot
    ├── DataPivotQuery
    └── DataPivotUpload

To this:

summary <django app>
└── Visual

One of the trickiest parts of the migration was migrating a few additional settings from DataPivot into the prefilters field in the Visual model; there's a little extra complexity in setting up the DataPivotQuery form that effectively unpacks and repacks this information when building the user-interface. I think/hope I added enough tests to cover the logic.

Data Pivot File are now migrated to datasets instead of a excel-file on a visual. Timestamps were preserved with the migration.

A few additional changes were also made with this migration:

• Change all forms in summary to use fields instead of exclude
• Added debug badge to many visual detail pages
• Added a URL redirect to go from old Data Pivot URL to the new visual-based URL
• Added faceted counts to django admin views for summary visuals and tables