Fixes and improvements (most related to visualizations)

1. Fix: when saving a query it was "hiding" all existing visualizations.
2. Fix: creating new query fails if user has old schema with visualizations.description non nullable.
3. Change chart type bar to column (there is no such thing bar).
4. Add scatter plot type.
5. Improve HighCharts performance (see: 8fbcd0c34dabd4c659f4d0e7614f8539ffe2da58). 
6. Bring back logging level setting.

... and some more.