config: move application_name to an initializer

It seems cleaner not to require a file before the Rails configuration
runs.

.rubocop.yml

@@ -20,7 +20,7 @@ DS/Unscoped:
 DS/ApplicationName:
   Enabled: true
   Exclude:
-    - './config/application_name.rb'
+    - './config/initializers/01_application_name.rb'
     - './lib/cops/application_name.rb'
     - './lib/linters/application_name_linter.rb'
     - "./spec/**/*"

config/application.rb

@@ -7,7 +7,6 @@ require 'rails/all'
 Bundler.require(*Rails.groups)
 
 Dotenv::Railtie.load
-require_relative 'application_name'
 
 module TPS
   class Application < Rails::Application

config/initializers/01_application_name.rb

@@ -1,3 +1,6 @@
+# This file is named '01-application-name.rb' to load it before the other
+# initializers, and thus make the APPLICATION_ constants available in
+# the other initializers.
 APPLICATION_NAME = ENV.fetch("APPLICATION_NAME", "demarches-simplifiees.fr")
 APPLICATION_SHORTNAME = ENV.fetch("APPLICATION_SHORTNAME", "d-s.fr")
 APPLICATION_BASE_URL = ENV.fetch("APPLICATION_BASE_URL", "https://www.demarches-simplifiees.fr")