Make karma load source maps from non-karma generated sources

When using webpack, we pack up everything outside of karma and
assuming you've set webpack to generate source maps it doesn't actually
use them.  This makes it so that it does use them so output can
be much better.

Test Plan:
   - Have your environment set up to use webpack
   - Change webpack.test.config.babel.js:23 to be:
         testWebpackConfig.devtool = 'inline-source-map';
   - Comment out lines 64-68 of the same file
   - Run npm run webpack-test-watch to bundle the
     tests
   - Run the specs (doc/testing_javascript.md)
   - Make a test fail... you should see a mapping
     from bundled test file to the actual file.

Change-Id: I650fe995d6aa595d89da5ae614d5407a699e75ec
Reviewed-on: https://gerrit.instructure.com/89822
Tested-by: Jenkins
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Clay Diffrient <cdiffrient@instructure.com>

karma.conf.js

@@ -80,6 +80,10 @@ var karmaConfig = {
 
   files: karmaFiles,
 
+  preprocessors: {
+    '**/*.js': ['sourcemap']
+  },
+
   proxies: {
     "/dist/brandable_css/": "/base/public/dist/brandable_css/"
   },

package.json

@@ -44,6 +44,7 @@
     "karma-phantomjs-launcher": "^1.0.0",
     "karma-qunit": "~1.1.0",
     "karma-requirejs": "^1.0.0",
+    "karma-sourcemap-loader": "^0.3.7",
     "karma-webpack": "^1.7.0",
     "on-build-webpack": "^0.1.0",
     "phantomjs-prebuilt": "^2.1.4",