In the cache, store the absolute path to the scs store

[jana-selva]

Saving the absolute path to the SCS file in cache instead of just the filename.
This way, we wouldn't need to define the relative path anymore in the notebooks.

The UI should still ask only for the (relative) filename.
Now the path should be taken from NOTEBOOKS environment variable:

Let's assume

• The user enters filename.sqlite into the UI dialog,
• The environment variable NOTEBOOK has the value /home/jupyter/notebooks
• Then the full path is

  /home/jupyter/notebooks/filename.sqlite