Multi-Currency Part 2

[zachgoll]

Overview

While the current app is built to support multi-currency, there are a few critical flows that still need to be implemented to support multi-currency.

This PR aims to introduce "v1" multi-currency support with:

• Money and Money::Currency classes (similar to money gem, but much simpler implementation tailored to our use case with major currency units and BigDecimal DB types)
• Support for all major currencies (see Money::Currency.all)
• Adds both a EUR and USD/EUR (multicurrency) account to both demo user and test fixtures
• Updates the account sync process to normalize historical account balances to a common currency defined by Family.currency
• Handles an "opt-in" exchange rates setup flow with Synth APIs for users who have accounts in multiple currencies
• Offers "on-demand" exchange rate syncs from Synth APIs

Remaining tasks

• Add exchange rates for fixtures
• Update account sync to convert balances to common Family.currency
• Update tests to expect multi-currency results
• Update all places in UI where multi-currency applies

[zachgoll]

Still a good number of follow-up PRs required beyond this one, but the core sync process for handling multi-currency accounts along with the associated tests are in.

[zachgoll]

Will revisit this after #553 is merged (lots of code overlap)