Notificaion options in create/edit Workflow modal
Add/fix workflow-specific notification settings

* Fix template logic for `notify_on_change` attribute
* Add migration and SQLAlchemy definition for `notify_on_change` and
  `notify_custom_message` attributes
Merge remote-tracking branch 'origin/develop' into feature/workflow-notification-ui

Conflicts:
	src/ggrc_workflows/models/workflow.py
Merge pull request #1536 from reciprocity/feature/workflow-notification-ui

Notification options in Workflow