Redesign file system infrastructure

Using this issue to track file system infrastructure work. Things to do:

[ ] Add export all (to zip) functionality.
[ ] Add import functionality
[ ] Redesign file loading UI so that loading is done on a separate page (ala Hackmd). The editor page would then only read a file from localstorage on initial load. This will likely alleviate some of the mysterious file deletion issues we have seen (mentioned above).
[ ] Add "last-n" snapshots of files to allow for rollback in the case of (accidental) deletion.
[ ] Add a locking mechanism to ensure that only one active page is editing a file at a time.

slag-plt / scamper