Add pagination to search

[mbklein]

There's some naming confusion here because (from previous commits) test-helpers.js injects a helpers object into the global context. But I've also just added src/helpers.js. Since the global helpers object doesn't exist at runtime, there's no concern about requiring it in any code under src, and so far I'm just using const { baseUrl } = require("path/to/helpers") in helpers.test.js to avoid a collision. But if the presence of both is awkward, I'm open to suggestions, e.g.,

• rename the global test-mode-only helpers object to testHelpers or _helpers or something else
• rename the new helpers.js file to something else (though we're already trying to avoid utils, but then again, is helpers any better?)