These config values are relatively static but in some cases they can be called many times in the same request (like rendering a report or a large list of line_items in BOM). These values will now only get fetched from Redis/Postgres once at most per request/job.
And mock the preference value, rather than setting it. Before, the set preference could have leaked to other tests.
(I noticed that it was already like this in product_import_spec.rb)
The DFC doesn't actually specify which cup it means. I don't expect
anyone providing "cup" as unit to measure their produce and expect it to
calculate as a regular volume. It can just be seen as items.
We still have the scale stored in two places but in our current system
that's part of a unit's "id".
If the DFC adds that value to its standard then we can use it for lookup
and don't need to repeat it.
* https://github.com/datafoodconsortium/taxonomies/issues/7
