Should give more nudging to save a copy on shared files

[jpolitz]

In CPO, when a shared file opens, various bad things can happen that could lose work.

For example, the user's computer or browser could crash, a data structures student could run the tab out of memory, and so on.

We have some stories where the likely case is that folks are editing shared files and then losing work, never having made their own copy. From their point of view, they lost work (even though we have various nudge warnings, etc, on closing the window, there are other ways for the work to go away).

We should either nudge to save, use localStorage somehow, or otherwise make this less likely to happen.