Notification/Alert when exit dashboard where changes were made

__Proposal:__
When closing (X) out Data Explorer or Dashboard cell creation, prompt user that changes have been made and will not be saved

__Current behavior:__
When exit dashboard after making changes in flux or my visualizations, closes out without any warning or prompt. With the X and check so close together, I have a couple times accidentally cancelled instead of saving.

__Desired behavior:__
Prompt "Do you want to save the changes you made? Changes will be lost if you don't save them" - can have the option to select if they would

__Alternatives considered:__

__Use case:__
Losing work done without any notice

No files or tests are named. Start by locating the Data Explorer and dashboard cell-creation close handlers, then trace how unsaved changes are tracked. Done means closing with X prompts before discarding edits while the existing save and cancel actions continue to work.

Written by the indexing model from the issue text.