Move Metadata to --internal database

[asg017]

Work-in-progress, not ready for review. refs #2341

• Removes .metadata() and ._metadata() fields from the Datasette class
• Removes /-/metadata.json endpoint
• Removes metadata "fallback"
• Removes the get_metadata() hook
• Adds the following methods to Datasette:
  • get_instance_metadata()
  • get_database_metadata()
  • get_resource_metadata()
  • get_column_metadata()
  • set_instance_metadata()
  • set_database_metadata()
  • set_resource_metadata()
  • set_column_metadata()
• Adds the following tables to internal.db
  • datasette_metadata_instance_entries
  • datasette_metadata_database_entries
  • datasette_metadata_resource_entries
  • datasette_metadata_column_entries

TODO

• Support metadata.json
• Don't change facet logic, move facet_size to synchronous config
• Docs!

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📚 Documentation preview 📚: https://datasette--2343.org.readthedocs.build/en/2343/