Implement a global setting for event fees in admin area

[mariobehling]

The current implementation of fees at https://eventyay.com/admin/settings/ticket-fees only works for specific currencies. Instead we need a global setting for all countries and currencies. Also fees should always depend on the country and not on the currency. Therefore please change the features as follows:

• Implement a global setting for ticket fees for all countries
• Make the country specific fees an exception of the global setting
• Take out field for "currencies". We only choose fees based on countries.