Add a more comprehensive system/code/display system

[mikix]

We have a few places where we manually convert a system/code to a display value (race, ethnicity, attachment format).

But it's all ad-hoc and could be more robust / easier to do for new systems. And we have several more places where we are showing codes but we should be using display values.

(I have avoided pulling the display value from the source, because (a) we want to allow better comparison between sites running this metric and (b) that field is properly allowed to be blank - I'd rather not rely on it.)

So for this ticket, maybe do a thing where we load display codes from a file into a table and join that. And then use that in more places.