help webpack know it can dead-code-eliminate ui-testable

Test plan:
* run webpack in prod mode
* you should not see the string “Could not append locator attribute”
  Anywhere in the public/dist/webpack-production folder
* the main webpack bundle should be smaller

Change-Id: Id5bf89d09d5e8c49c01da24780d792834143e534
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/214121
Tested-by: Jenkins
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

frontend_build/baseWebpackConfig.js

@@ -299,7 +299,8 @@ module.exports = {
     new webpack.EnvironmentPlugin({
       NODE_ENV: null,
       DEPRECATION_SENTRY_DSN: null,
-      GIT_COMMIT: null
+      GIT_COMMIT: null,
+      ALWAYS_APPEND_UI_TESTABLE_LOCATORS: null
     }),
 
     // Only include timezone data starting from 2011 (canvaseption) to 15 years from now,