[UX] Make it easy to find newly-added modules on the modules page

As a follow-up to https://github.com/backdrop/backdrop-issues/issues/494#issuecomment-6787...

Proposals for how to indicate that a module is new: - add a JS toggle (checkbox) for "Show new modules" - list the new modules again, separately, at the very top of the page

I was working on this same thing in D8 and here are the related issues: https://www.drupal.org/node/1355526 (which we should already have in Backdrop) https://www.drupal.org/node/538904 (no real ocnsensus on filter vs grouping for new)