Add npm to dependabot monitoring

This is so that dependabot can raise PRs for changes in dependencies
referenced in the package.json file, which is currently just jQuery.

My motivation for adding this is noticing we weren't getting dependabot
PRs to update jQuery from 1.12.

.github/dependabot.yml

@@ -5,6 +5,11 @@ updates:
     schedule:
       interval: daily
 
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: daily
+
   # Ruby needs to be upgraded manually in multiple places, so cannot
   # be upgraded by Dependabot. That effectively makes the below
   # config redundant, as ruby is the only updatable thing in the