Add .buildpacks for heroku-buildpack-multi
Remove webassets node binary paths in production

Our common production deployment assumes a heroku or heroku-like
environment. In this environment, the PATH is already set from the
buildpack.
node deps are not development only

Until we start packaging and releasing a version with pre-built
assets, these are needed at runtime.
Add node engines to package.json
Add/Hack a script to autobuild Firefox extension

Like with the bookmarklet, the extension is created for the currently used
domain name. The building is only done the first time /hypothesis.xpi is
visited.

This code is very fragile and ugly, don't use it for anything serious!
Add annotator-store/reddit-proxy as requirement
Switch back to annotator index in elasticsearch