"Save" button should save all datasets

[travs]

I digitized a few plots today and assumed that clicking "save" when I was done each figure was saving all of the datasets, when in fact it only saved the last one. I realized this too late, and lost a lot of that work.

The save button is kind of hidden (you have to click "view data" first), and must be clicked for every dataset, which is easy to forget when there are 5 or 6 datasets to deal with. It would be better if it was in the open and hard to miss, or if it autosaved your session automatically, and just saved to the db when you click save.

Anyway, I wrote it up in the new changes to the docs, and I will try not to be so naïve about save functionality in the future.

cc: @miladjafary @VahidGh