Add Expense Fields

[bradley-holt]

Please add a "Travel Expenses" (travel_expenses) and a "Non-Travel Expenses" (non_travel_expenses) number field to events, along with a currency indicators for these fields (travel_expenses_currency and non_travel_expenses_currency) with the options of USD, CAD, GBP, and EUR (likely to be expanded in the future). Please also add a description field for "Non-Travel Expenses" (non_travel_expenses_description). These fields will be used to record the total per Developer Advocate travel and non-travel expenses for a specific event, as indicated on the associated expense report.

[bradley-holt]

@glynnbird Looking good. Two minor remaining TODOs on this:

1. We need a description field for "Non-Travel Expenses" (non_travel_expenses_description).
2. We don't need it right now, but it's possible that our friends to the north (Canada) could use this and would need a CAD currency option.