Store successful donations in local storage

[LeafHacker]

Currently, a user navigating away from an in-progress or completed donation will result in the registration token being lost. #66 will help mitigate this issue, but it'd be great if supported browsers could show recent donation tokens using local storage.

[LeafHacker]

A script in api.js could call the check token endpoint for each cached token and delete any used ones. This could be run onload by the donate & register forms.

We could even make the check token endpoint support checking a list of tokens, although idk if this presents a potential security risk without harsh rate limits.