Multi-currency support: Money + Currency class improvements

[zachgoll]

This PR is a precursor to #543, which was growing too big and tackling too many things at once.

Overview

Below are some explanations of changes and sample usage for documentation purposes.

Money lib

Adds Money, Money::Currency, and Money::Arithmetic to lib as these represent generic concepts that could apply to any app that deals with money.

• Money packages up all the information required to represent a monetary value (amount, currency)
• Money::Currency stores information about all global currencies and can provide a list of these through Money::Currency.all or Money::Currency.popular
• Money::Arithmetic allows us to add, subtract, multiply, divide, and compare monetary values without any special syntax. In other words, we can do Money.new(0) == Money.new(0) or Money.new(100) > Money.new(5)

Why not money and money-rails?

There are two main reasons I decided to go with a custom implementation:

1. The money and money-rails gems are somewhat opinionated about a project's money storage strategy. Throughout most of its API, it assumes that money is being stored in minor currency units. Our project does not do that. We store in Decimal{19,4}, which was chosen to make the data a lot more intuitive (1 less layer of indirection). Furthermore, the money-rails gem integrates at the model level (with migration helpers), which as we saw with a first attempt at using it, caused unnecessary confusion for new contributors.
2. The overall implementation here is fairly simple, so we get the benefit of owning our implementation without a huge maintenance cost.

Monetizable Concern + Form Helper

In an attempt to keep things simple and intuitive, as an alternative to the money-rails strategy that "magically" turns fields like Account.balance into Money instances, I've created a Monetizable concern with a straightforward API:

class Account
  include Monetizable

  monetize :balance # creates `balance_money` getter method, which returns instance of Money
end

This unobtrusively adds a field called balance_money, which returns Money.new(balance, currency).

Furthermore, I've created a form helper with the following API:

<%= form_with model: @transaction do |f| %>
  <%= f.money_field :amount_money %>
  <%= f.submit %>
<% end %>

f.money_field will read amount_money and create an :amount and :currency input that is updated on submission.

The overall goal here is to give the developer flexibility when working with money and making it explicit when a field deals with money and when it doesn't.