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

.rubocop_todo.yml

@@ -307,7 +307,7 @@ Style/FetchEnvVar:
     - 'config/initializers/devise.rb'
     - 'config/initializers/paperclip.rb'
     - 'config/initializers/vapid.rb'
-    - 'lib/mastodon/premailer_webpack_strategy.rb'
+    - 'lib/premailer_webpack_strategy.rb'
     - 'lib/mastodon/redis_config.rb'
     - 'lib/tasks/repo.rake'
     - 'spec/features/profile_spec.rb'

config/initializers/premailer_rails.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative '../../lib/mastodon/premailer_webpack_strategy'
+require_relative '../../lib/premailer_webpack_strategy'
 
 Premailer::Rails.config.merge!(remove_ids: true,
                                adapter: :nokogiri,