Save answers on back button

[BryceStevenWilley]

"Back" shouldn't be "Undo", straight up. The chunk that is un-done by a click of the back button should be stored elsewhere in the interview (one item per variable, you might still lose generic or repeated answers), and if the interview progresses again, it should be used as default.

IMO critical to simple usability

Somewhat related to #2, in that we'd need a mechanism to store answers in an interview that aren't "answered" perse.

[BryceStevenWilley]

Relevant code: https://github.com/jhpyle/docassemble/blob/d6cd734215ea48c8a29eb5dfc8382b669fa7908a/docassemble_webapp/docassemble/webapp/server.py#L6987-L6999 happens when you click "Back". Notably:

• fetch_previous_user_dict is called. This deletes the highest UserDict.indexno for this user on that interview, and fetches it like normal.
• sets the current interview status based on the old dict and stuff.

Need to figure out what happens with old_user_dict in the rest of index(). But I still think it's possible to get all of the key-values in old_user_dict and not in user_dict, store them somewhere, and if a page with that variable shows up as an input (i.e. not continue button or anything), then it can use that value as default.

For SMS, this happens in line 25504 of server.py. Not sure how to handle that though.