Add option to save edit tokens in local storage

[PurkkaKoodari]

Currently, edit tokens are only sent over email. It would make sense to allow them to be saved in the browser session as well, so there could be an edit button directly on the event page.

However, I think we want to also allow the user to not store the token, e.g. if on a shared computer. I would implement this as a checkbox on the signup editor, and also persist that checkbox in local storage. I'd say we should save tokens by default.