Autostart URLs should allow to directly open a notebook

[lorenzo-cavazzi]

Motivation

We already open JupyterHub sessions directly into a notebook from the file viewer.
Some users expressed interest in having quickstart links for the sessions doing the same thing (@rokroskar ).

Proposal

Technically it is not too difficult, although this may need some coordination with renku-notebooks since the current mechanism is based on JupyterLab, and that may not be there anymore for R-based images.

Add a button to the File view, when the user is looking at a notebook to create a sharable link to start the notebook in a session:

This probably belongs to the "Create shareable link" modal. An idea could be adding an expandable menu that allows selecting (through a radio button) a target notebook to open.
When clicking on [set default notebook], the UI should scan the files in the project and identify the notebooks, listing them (verify the feasibility here for projects containing thousands of files). The selection should always include also none to reset a previous choice.

Context

[this is the current modal, not a mockup]

[rokroskar]

Thanks @lorenzo-cavazzi for writing this up! I think the proposal makes sense. cc @volodymyrss