Store currency per account #64

Outcome. A collector chooses the currency their prices paid are shown in.

• Currency is an instance setting today (spec 004, config/initializers/currency.rb). It is a collector's choice, so it is the one known breach of hosted-door rule 3.
• Collector-owned data: extends the export.
• Needs a migration that carries the instance's current currency to every account.

A candidate from the roadmap. It is refined into a spec when its turn comes, and checked then against the north star's hosted-door rules and spec rules (one outcome, one PR; a spec that adds collector-owned data extends the export in the same PR).