Replace Should.js with Chai

[matthew-white]

Should.js hasn't been updated on npm since 2018, and its repo is archived. We had to move from Should.js to Chai in Frontend (getodk/central-frontend#1015). There are a lot of similarities between the two, but Chai is better documented. There are also some differences, and I think trying to remember those differences while switching between Backend and Frontend will end up causing friction. This issue is to replace Should.js with Chai in Backend. We should time work on this issue such that no large Backend work is in progress.