Use stable-module-id-hash webpack plugin

[KyleAMathews]

#533 worked to make module ids stable across builds but added the complication that users needed to checkin the records.json files that Webpack outputs. Which is confusing.

This PR adds https://github.com/zhenyong/webpack-stable-module-id-and-hash which replaces the default incrementing int module ids from Webpack core with 8 digit module ids derived from the hash of each module.

[KyleAMathews]

Hmmm... stable-module-id-hash is throwing a collision error on every build on Netlify. Created issue at zhenyong/webpack-stable-module-id-and-hash#8.

In the meantime let's just use the named module plugin which also is deterministic until the above bug is fixed. It's not ideal putting named modules in production as this can leak information but since we're in Alpha no big deal :-) Also going to switch to Webpack 2 soon which has its own hashedmoduleid plugin.