Re-link missing notebooks

[cameron-toy]

Currently, loading a dashboard when a required notebook is missing causes the outputs from the missing notebook to be red, and attempting to load a cell that doesn't exist from a valid notebook causes the output to be yellow. This isn't very useful except to show that something is wrong, and doesn't offer any solutions.

A more useful solution would be to store the Base64 encoding of the output when the dashboard is saved (similar to how notebooks save the last output of each cell) so that the user can see a preview of the missing outputs. Also, dragging a notebook from the JupyterLab file browser on to a missing widget could re-link the notebook to the dashboard, updating its path.

Mock up:

[juliahuynh]

Flows for user relinking missing notebooks

User chooses to relink when warning dialog appears

User chooses to ignore when warning dialog appears