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).
* Web interface to add and delete data sources, without the need to ssh
into the server.
* Ability to safely delete datasources -- query results from this data sources
are deleted, while queries get assigned null datasource.
* Updated the BigQuery datasource to use the JSON key file from Google Developer
console. Also both BigQuery and the Google Spreadsheets datasource no longer store
their key on the filesystem, but rather in the DB.
* Minor updates to the Flask Admin.
