Add model(s) to persist embed config

[jwalgran]

Overview

The embed config is complex and will grow over time as we add features. Rather than serialize to the query string, store the config in the database and link it to the contributor.

Describe the solution you'd like

• Add EmbedConfig
  • Add nullable embed_config OneToOne FK to the Contributor model.
  • Fields
    • Width, height strings
    • color string nullable
    • font string nullable
    • show other contributor information bool default false
• Add EmbedFields
  • unique_together on embedconfig id and order
  • FK to EmbedConfig
  • column_name string
  • display_name string
  • visible bool default false
  • order number
• Add CRUD APIs for embed config
  • For EmbedFields delete all existing then create new rows for the updated list
  • Use as much automatic DRF functionality as possible