[catalog-server] Add a ranking metric

We need a default way to rank elements in queries or just browsing the catalog.

There are a number of metrics we could pull from:
- npm downloads
- GitHub stars
- Last publish date
- Catalog page views
- Manifest completeness and validation
- package.json completeness and validation
- Number of dependents in npm
- Ratings in the catalog itself

Some of these would require writing back to the catalog database, some would require a log-in. So we may want to avoid those.

npm divides these metrics into popularity, quality, and maintenance, which might be a useful way to think about these. It's probably not that useful to allow custom mixing of those metrics like npm allows, but we could internally group metrics into those categories before mixing to rationalize the weights we give different metrics.

Most of these metrics are also per package, not per element, so we run the risk of over-weighting rarely-used/low-quality elements in otherwise high-quality/popularity packages. The only way to address that is with per-element measures like star ratings in the catalog or manifest completeness per element.