FIX: Make all email subject vars available in notification subjects (#11064)

A site owner attempting to use both the email_subject site setting and translation overrides for normal post notification
email subjects would find themselves frusturated at the lack of template argument parity.
Make all the variables available for translation overrides by adding the subject variables to the custom interpolation keys list and applying them.

Reported at https://meta.discourse.org/t/customize-subject-format-for-standard-emails/20801/47?u=riking

app/models/translation_override.rb

@@ -9,6 +9,13 @@ class TranslationOverride < ActiveRecord::Base
       topic_title_url_encoded
       site_title_url_encoded
       context
+
+      site_name
+      optional_re
+      optional_pm
+      optional_cat
+      optional_tags
+      topic_title
     }
   }
   include ActiveSupport::Deprecation::DeprecatedConstantAccessor

lib/email/message_builder.rb

@@ -53,7 +53,15 @@ module Email
     def subject
       if @opts[:template] &&
           TranslationOverride.exists?(locale: I18n.locale, translation_key: "#{@opts[:template]}.subject_template")
-        subject = I18n.t("#{@opts[:template]}.subject_template", @template_args)
+        augmented_template_args = @template_args.merge({
+          site_name: @template_args[:email_prefix],
+          optional_re: @opts[:add_re_to_subject] ? I18n.t('subject_re') : '',
+          optional_pm: @opts[:private_reply] ? @template_args[:subject_pm] : '',
+          optional_cat: @template_args[:show_category_in_subject] ? "[#{@template_args[:show_category_in_subject]}] " : '',
+          optional_tags: @template_args[:show_tags_in_subject] ? "#{@template_args[:show_tags_in_subject]} " : '',
+          topic_title: @template_args[:topic_title] ? @template_args[:topic_title] : '',
+        })
+        subject = I18n.t("#{@opts[:template]}.subject_template", augmented_template_args)
       elsif @opts[:use_site_subject]
         subject = String.new(SiteSetting.email_subject)
         subject.gsub!("%{site_name}", @template_args[:email_prefix])

spec/components/email/message_builder_spec.rb

@@ -258,17 +258,28 @@ describe Email::MessageBuilder do
     end
 
     context "when use_site_subject is true" do
-      let(:templated_builder) { Email::MessageBuilder.new(to_address, template: 'mystery', use_site_subject: true) }
+      let(:templated_builder) { Email::MessageBuilder.new(to_address, template: 'user_notifications.user_replied', use_site_subject: true, topic_title: "Topic Title") }
 
       it "can use subject override" do
-        override = TranslationOverride.create(
-          locale: I18n.locale,
-          translation_key: "mystery.subject_template",
-          value: "my customized subject"
+        override = TranslationOverride.upsert!(
+          I18n.locale,
+          "user_notifications.user_replied.subject_template",
+          "my customized subject"
         )
-        I18n.expects(:t).with("mystery.subject_template", templated_builder.template_args).returns(override.value)
+        override.save!
         expect(templated_builder.subject).to eq(override.value)
       end
+
+      it "can use interpolation arguments in the override" do
+        SiteSetting.email_prefix = 'some email prefix'
+        override = TranslationOverride.upsert!(
+          I18n.locale,
+          "user_notifications.user_replied.subject_template",
+          "[%{site_name}] %{topic_title} my customized subject"
+        ).save!
+        expect(templated_builder.subject).to match("some email prefix")
+        expect(templated_builder.subject).to match("customized subject")
+      end
     end
 
   end