Exchange Rates don't actually refresh

[madsobel]

I just realized that exchange rates dosen't actually refresh periodically, and are only fetched during application start up. This is a huge oversight on my part and should be fixed.

As well as actually refreshing them we should update the documentation to make it clear when rates are updated according to:
https://www.ecb.europa.eu/stats/policy_and_exchange_rates/euro_reference_exchange_rates/html/index.en.html