Add backup reminder

[MartijnR]

It is possible for a user to delete localStorage by clearing the browser cache (with a particular option ticked in most browsers). This will permanently delete all local records.

It may be good to add a backup reminder to the interface that prompts under the following conditions:

• there is at least one record in localStorage
• it hasn't prompted before that day

It could use a 5 minute continuous loop checking these conditions, starting 5 minutes after page load.