* feat: add support for alerts
* Add ModelViews
* First pass at adding scheduled Alert support
* Fix syntax errors
* Add log_retention and database to model
* Improving screenshots
* Still refactoring screenshots
* Pass down thumb_size properly
* Progress on screenshot endpoint
* Add alerts.last_eval_dttm and alert query logic
* Getting ready to split compute_screenshot and screenshot/{SHA}
* split the endpoints
* Improving alerting loop
* empty methods
* Add CLI command 'superset alert' that runs a sched loop
* Getting things to work
* email delivery
* A working email\!
* Add feature flag
* Add some description_columns to AlertModelView
* Little tweaks
* Use database.get_df, eval cells for truthiness
* Migrate thumbnail/screenshot functions to use new arg pattern
* Addressing PR feedback
* Update alerts DB migration down_revision
* Resolve _deliver_email arg conflict
* Make mypy happy
* Make isort happy
* Make pylint happy
Co-authored-by: Rob DiCiuccio <rob.diciuccio@gmail.com>
