Disable source maps in production (v2)

[tomazy]

Fixes #3817

[KyleAMathews]

Thanks for jumping on this!

[KyleAMathews]

The API for modifying webpack config has changed. Instead of directly mutating the webpack config, there's a new action named setWebpackConfig. If you grep the v2 branch, you'll find a number of usages for this + I think there's docs already.

[tomazy]

Ok, fixed.

[KyleAMathews]

Hmmmm... so I might have changed my mind about this. I agree with @jquense (#3817 (comment)) that they're more useful than not. Instead of disabling it by default — how about instead we a) investigate using relative paths, removing comments and otherwise making them more tidy by default and b) add a plugin to easily disable producing source maps by modifying the webpack config which also conveniently would speed up builds some.