feat: exchange rate provider bitstamp_eur

[m-schmoock]

This simply adds a bitstamp_eur provider that can also be set as environment variable.

Update: this is only half of the work, as we also need to care the client code that renders '$' or 'usd' when prices are shown.

[m-schmoock]

Note: CI seems broken, but unrelated to this PR:

E: Version '8u265-b01-0+deb9u1' for 'openjdk-8-jdk-headless' was not found
The command '/bin/sh -c apt-get install -y --no-install-recommends openjdk-8-jdk-headless=8u265-b01-0+deb9u1' returned a non-zero code: 100