This should allow us to define and update our dependencies more easily (via npm) as well as reducing the number of requests.
Additionally, when we add common packages this will avoid the issue after deploys where a user may get a cached HTML page, but new Javascript, leading to the dreaded blank page because the browser didn't download all dependencies.
Using mangle with UglifyJS also made the total file size for the common chunk bundle smaller than the current total for all of our external/polyfill scripts (1.18MB vs 1.55MB).
Assumes the session cookie is stored as JSON which may or may not have been compressed via zlib (indicated by a leading `.`), which is then base64-encoded, and made URL-safe by replacing all `+` and `/` characters with `-` and `_` respectively.
