Vis index API endpoint tests and new shared filter

- `/api/v1/viz/` has flags for `exclude_shared` and `only_shared`. Make sure all scenarios work as intended both for `type=table` (tables/datasets) and `type=derived` (visualizations).
- Added convenience filter `shared` with values so frontend only handles one flag:
  * `yes`: Shared and non shared items
  * `no`: Only non-shared items
  * `only`: Only shared items

https://github.com/CartoDB/cartodb/issues/1592