Rspec feature tests have useless javascript stack traces

When the webpack.config.js was updated in !11815 (merged) it removed the default sourcemap configuration from the webpack compile process. When run with dev-server, or karma, or when compiled for production there would be a sourcemap config, but not when compiled for testing with yarn webpack.

We need to restore this so that we can more accurately debug our frontend code when rspec fails in CI.

Update:

It seems that although it is true we don't generate sourcemaps for rspec tests in CI, it doesn't matter much because adding them back doesn't produce anything more useful. This appears to be a limitation of PhantomJS or of Capybara/Poltergeist. Strangely our stack traces worked fine with PhantomJS when run through Karma. Currently marking as blocked until #30876 (moved) is resolved.