Require packages to be downloaded >= 100x/day

This should make it more difficult for people who are publishing
malicious packages to npm to get them on the CDN.

server/middleware/checkMinDailyDownloads.js

@@ -0,0 +1,20 @@
+const NPMDownloads = require('../NPMDownloads')
+
+function checkMinDailyDownloads(minDailyDownloads) {
+  return function (req, res, next) {
+    NPMDownloads.getDaily(req.packageName, function (error, downloads) {
+      if (error) {
+        console.error(error)
+        next() // Keep going; this error isn't critical.
+      } else if (downloads == null) {
+        res.status(404).type('text').send(`Cannot find package "${req.packageName}"`)
+      } else if (downloads >= minDailyDownloads) {
+        next()
+      } else {
+        res.status(404).type('text').send(`Cannot serve requests for package "${req.packageName}" because it has been downloaded on average only ${downloads} time${downloads > 1 ? 's' : ''} per day this week`)
+      }
+    })
+  }
+}
+
+module.exports = checkMinDailyDownloads