This is one huge change for the permissions system and related:
* (Backward incompatible:) Remove the table based permissions in favour of the new model.
* Manage permission to view or query datasources based on groups.
* Add the concept of Organization. It's irrelevant for most deployments, but allows for
multi-tenant support in re:dash.
* Replace ActivityLog with Event based rows (old data in activity_log table is retained).
* Enforce permissions on the server-side. There were some permissions that were only enforced
on the client side. This is no more. All permissions are enforced by the server.
* Added new permission: 'super-admin' to access the status and Flask-Admin interface.
* Make sure that html is never cached by the browser - this is to make sure that the browser
will always ask for the new Javascript/CSS resources (if such are available).
