Resolve custom application stylesheet with Webpack (#3373)

This implementation is a bit smaller and still has the following benefits:

* No need of app/javascript/packs/custom.js
For custom stylesheet, it typically has only
"require('../styles/custom.scss')" and is redundant.

* No need to extract vendor stylesheet to another asset
Extracting vendor stylesheet could be forgotten by developers who do not
use custom stylesheet.

app/helpers/style_helper.rb

@@ -1,17 +0,0 @@
-# frozen_string_literal: true
-
-module StyleHelper
-  def stylesheet_for_layout
-    if asset_exist? 'custom.css'
-      'custom'
-    else
-      'application'
-    end
-  end
-
-  def asset_exist?(path)
-    true if Webpacker::Manifest.lookup(path)
-  rescue Webpacker::FileLoader::NotFoundError
-    false
-  end
-end

app/javascript/mastodon/main.js

@@ -2,7 +2,7 @@ const perf = require('./performance');
 
 // import default stylesheet with variables
 require('font-awesome/css/font-awesome.css');
-require('../styles/application.scss');
+require('mastodon-application-style');
 
 function onDomContentLoaded(callback) {
   if (document.readyState !== 'loading') {

app/views/layouts/application.html.haml

@@ -18,8 +18,7 @@
         = ' - '
       = title
 
-    = stylesheet_pack_tag 'common', media: 'all'
-    = stylesheet_pack_tag stylesheet_for_layout, media: 'all'
+    = stylesheet_pack_tag 'application', media: 'all'
     = javascript_pack_tag 'common', integrity: true, crossorigin: 'anonymous'
     = javascript_pack_tag "locale_#{I18n.locale}", integrity: true, crossorigin: 'anonymous'
     = csrf_meta_tags