Build: Restore the external directory

In gh-4466, we removed the `external` directory in favor of loading some files
directly from `node_modules`. This works fine locally but when deploying code
for tests, this makes it impossible to not deploy `node_modules` as well. To
avoid the issue, this change restores usage of the `external` directory.

One change is that we no longer commit this directory to the repository, its
only purpose is to have clear isolation from `node_modules`.

Ref gh-4466
Closess gh-4865

.npmignore

@@ -7,6 +7,7 @@
 /.travis.yml
 
 /build
+/external
 /speed
 /test
 /Gruntfile.js