Create family currency migration flow

[zachgoll]

As of #1626, users can no longer change their family currency after the onboarding step has been completed. This is to prevent confusion caused by legacy transactions, accounts, and budgets that were in the prior family currency. Since pretty much every calculation in the app revolves around this, it's not straightforward to just update it.

Long term, we need to support a change like this because some users may have valid reasons for changing their preferred family currency.

Potential flow

1. User updates family currency
2. Maybe shows a "takeover" that let's them know their data is being migrated
3. Background job fires, performs data updates needed (including currency conversions)
4. Once complete, user can interact with app again