Better currency support

[lroellin]

Currently, the only currencies are CHF and EUR. However, they're just display values and nothing changes when you change it.
We could offer a currency changer here, where you could switch currencies in one go. This would go like this (API actions in bold, everything else is out-of-scope in this repo)

1. The client changes the currency
2. The client sends an exchange value between the two currencies. UI-wise, the user could be shown a default value that the client gets from a monetary exchange API
3. The backend converts all amounts according to that exchange value

(if we want to go with that route, we can create issues for web.teiler.io for an example implementation)