* [scheduled reports] Add support for scheduled reports
* Scheduled email reports for slice and dashboard visualization
(attachment or inline)
* Scheduled email reports for slice data (CSV attachment on inline table)
* Each schedule has a list of recipients (all of them can receive a single mail,
or separate mails)
* All outgoing mails can have a mandatory bcc - for audit purposes.
* Each dashboard/slice can have multiple schedules.
In addition, this PR also makes a few minor improvements to the celery
infrastructure.
* Create a common celery app
* Added more celery annotations for the tasks
* Introduced celery beat
* Update docs about concurrency / pools
* [scheduled reports] - Debug mode for scheduled emails
* [scheduled reports] - Ability to send test mails
* [scheduled reports] - Test email functionality - minor improvements
* [scheduled reports] - Rebase with master. Minor fixes
* [scheduled reports] - Add warning messages
* [scheduled reports] - flake8
* [scheduled reports] - fix rebase
* [scheduled reports] - fix rebase
* [scheduled reports] - fix flake8
* [scheduled reports] Rebase in prep for merge
* Fixed alembic tree after rebase
* Updated requirements to latest version of packages (and tested)
* Removed py2 stuff
* [scheduled reports] - fix flake8
* [scheduled reports] - address review comments
* [scheduled reports] - rebase with master
* Template dashboard
* Fix MySQL test
* Model for user attributes
* Redirect to welcome dash
* Fix lint
* Add missing file
* Add migration script
* Fix lint
* Fix more lint
