Allow users to store their Akita data to their browser account

[sharon-wang]

Users may want to see their congregated Akita data across browser instances, i.e. on different devices. We should give users the option to store their data to their browser account so they can access it on various devices, instead of seeing different totals on different devices. We can also allow users to see how their payments different across browser instances.

However, we've made a conscious choice to avoid sending any user data anywhere (everything stored locally only), so this change would contradict that choice.