Move lib/mastodon/premailer_webpack_strategy to lib/ (#27636)

lib/premailer_webpack_strategy.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module PremailerWebpackStrategy
+  def load(url)
+    asset_host = ENV['CDN_HOST'] || ENV['WEB_DOMAIN'] || ENV['LOCAL_DOMAIN']
+
+    if Webpacker.dev_server.running?
+      asset_host = "#{Webpacker.dev_server.protocol}://#{Webpacker.dev_server.host_with_port}"
+      url        = File.join(asset_host, url)
+    end
+
+    css = if url.start_with?('http')
+            HTTP.get(url).to_s
+          else
+            url = url[1..] if url.start_with?('/')
+            Rails.public_path.join(url).read
+          end
+
+    css.gsub(%r{url\(/}, "url(#{asset_host}/")
+  end
+
+  module_function :load
+end