Add Subresource Integrity to Snippet

[drewmendenhall]

This change protects the loading of rollbar.js via the Browser JS Snippet from the cdn attack vector via Subresource Integrity.

I made a few changes to the build system in order to make this happen:

• The webpack config that builds the snippet has been broken out into a separate config file. This config also exports a function so that the manifest can be read later in the build pipeline.
• webpack-assets-manifest outputs the sha256 hash of the file which ends up on the cdn (minified vanilla config) to a file dist/manifest.json
• Grunt builds rollbar.js via the webpack node api, not the grunt-webpack. This is done because the snippet build depends on the assets output from the vanilla build.
  • I'm pretty rusty with Grunt, so I couldn't find a good way to get things to work "the grunt way". I can put up another WIP branch if you'd like.

It might be nice to leverage multi-compiler mode in order to avoid reading from disk. Also, I'm not sure if it's useful to publish dist/manifest.json.