* Process extra column metadata for a few sql-based data sources.
* Add Table and Column metadata tables.
* Periodically update table and column schema tables in a celery task.
* Fetching schema returns data from table and column metadata tables.
* Add tests for backend changes.
* Front-end shows extra table metadata and uses new schema response.
* Delete datasource schema data when deleting a data source.
* Process and store data source schema when a data source is first created or after a migration.
* Tables should have a unique name per datasource.
* Addressing review comments.
* Update migration file for mixins.
* Appease PEP8
* Upgrade migration file for rebase.
* Cascade delete.
* Adding org_id
* Remove redundant column and table prefixes.
* Non-existing tables and columns should be filtered out on the server side not client side.
* Fetching table samples should be optional and should happen in a separate task per table.
* Allow users to force a schema refresh.
* Use updated_at to help prune old schema metadata periodically.
* Using settings.SCHEMAS_REFRESH_QUEUE
