The `/ballot` page is running 6 SQL queries per ballot entry

[dfabulich]

• Add a test asserting that adding entries doesn't cause N queries #487

Run the test in #487.

Actual: The test fails, saying that adding 15 entries adds 90 SQL queries to the page, 6 queries per entry.

Expected: The test should pass, saying that adding 15 entries doesn't add 15 SQL queries to the page.

The ballot should query for all needed data up front, using DBIC joins and prefetching.