Require any modules after loading polyfill in entry points (#4231)

app/javascript/mastodon/main.js delayed the execution of modules,
but other entry points didn't. That leads to failure in executing
modules, which requires those polyfills.

Strictly enforce the rule to require any modules after loading
polyfill in entry points.

app/javascript/packs/application.js

@@ -1,6 +1,7 @@
-import main from '../mastodon/main';
 import loadPolyfills from '../mastodon/load_polyfills';
 
-loadPolyfills().then(main).catch(e => {
+loadPolyfills().then(() => {
+  require('../mastodon/main').default();
+}).catch(e => {
   console.error(e);
 });