Celery-director

Changed
- Upgrade Celery and Kombu packages for bug fixes (celery 4.4.0 to 4.4.7, kombu 4.6.7 to 4.6.11)

Fixed
- Roll back the session when an unhandled exception occurred during the request

Added
- Add the support of celery crontab scheduler
- Add auto refresh workflows list in director home page

Fixed
- Add type deletion during downgrade
- Fix mysql string column length (database upgrade required: `director db upgrade`)

Added
- Add an example about a group of tasks
- Add index on workflow_id field in tasks table

Changed
- Rename default workflow to example.ETL

Fixed
- Fix issue with icons when using the `DIRECTOR_ENABLE_CDN=false` and `director dlassets` command

Added
- Add Sentry error reporting (use `DIRECTOR_ENTRY_DSN` setting)
- Add GitHub link in the Web UI header

Fixed
- Fix WebUI workflow table.
- Fix WebUI task color status for `progress`

Changed
- Limit search placeholder on dates and IDs

Fixed
- Handle the default pagination setting

Changed
- New interface based on d3.js
- Support the mode history in permalinks