Optimize status page loading

The status page currently does N+1 queries to load the history of artifact collections. It would be better to write a specific query that will load the last N collections together with their errors and PR numbers.

The code for the status page is [here](https://github.com/rust-lang/rustc-perf/blob/b66899a5a70feab666fee188b07f31f0aa529a24/site/src/request_handlers/status_page.rs#L41).