set publicPath ui-build/webpack/index.js

this resolves some chunk errors in dev

test plan:
  - build passes

flag=none

Change-Id: I61dcc44b214374d5f2ec43c790c1244fdb7eacbc
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/327912
Reviewed-by: Isaac Moore <isaac.moore@instructure.com>
Build-Review: Isaac Moore <isaac.moore@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Aaron Shafovaloff <ashafovaloff@instructure.com>
Product-Review: Aaron Shafovaloff <ashafovaloff@instructure.com>

ui-build/webpack/index.js

@@ -139,7 +139,8 @@ module.exports = {
   watchOptions: {ignored: ['**/node_modules/']},
 
   output: {
-    publicPath: '',
+    publicPath:
+      process.env.NODE_ENV !== 'production' ? '/dist/webpack-dev/' : '/dist/webpack-production/',
     clean: true, // clean /dist folder before each build
     path: join(canvasDir, 'public', webpackPublicPath),
     hashFunction: 'xxhash64',