R1:1e74c05ac6ff

Add timestamps to development-mode static resource URIs

Summary:
When a developer changes CSS, it is normally sufficient to reload the page to get changes to show up, because browsers revalidate resources on reload.

However, if you reload the page and then an Ajax request adds new CSS to the page, this CSS does not trigger revalidation. The developer must currently clear their cache or re-run `scripts/celerity_mapper.php webroot`, to get this request to skip cache. We rarely use CSS over Ajax right now…