Fix premailer and S/MIME emailer hooks order

Premailer must run before the S/MIME interceptor or signatures will be
mangled

changelogs/unreleased/fix-run-smime-email-interceptor-last.yml

+---
+title: Fix premailer and S/MIME emailer hooks order
+merge_request: 23293
+author: Diego Louzán
+type: fixed

config/initializers/action_mailer_hooks.rb

@@ -11,6 +11,9 @@ ActionMailer::Base.register_interceptors(
 
 ActionMailer::Base.register_observer(::Gitlab::Email::Hook::DeliveryMetricsObserver)
 
+# Force premailer loading so that it's not configured to run after the S/MIME interceptor
+::Premailer::Rails.register_interceptors
+
 if Gitlab.config.gitlab.email_enabled && Gitlab.config.gitlab.email_smime.enabled
   ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::SmimeSignatureInterceptor)
   Gitlab::AppLogger.debug "S/MIME signing of outgoing emails enabled"

spec/initializers/action_mailer_hooks_spec.rb

@@ -35,8 +35,11 @@ describe 'ActionMailer hooks' do
           load Rails.root.join('config/initializers/action_mailer_hooks.rb')
 
           if smime_interceptor_enabled
+            # Premailer must be registered before S/MIME or signatures will be mangled
             expect(ActionMailer::Base).to(
-              have_received(:register_interceptor).with(Gitlab::Email::Hook::SmimeSignatureInterceptor))
+              have_received(:register_interceptor).with(::Premailer::Rails::Hook).ordered)
+            expect(ActionMailer::Base).to(
+              have_received(:register_interceptor).with(Gitlab::Email::Hook::SmimeSignatureInterceptor).ordered)
           else
             expect(ActionMailer::Base).not_to(
               have_received(:register_interceptor).with(Gitlab::Email::Hook::SmimeSignatureInterceptor))