FIX: customized email subjects was ignored for some notifications

Email templates for notifications about new posts and topics weren't
using customized values.

lib/email/message_builder.rb

@@ -51,7 +51,10 @@ module Email
     end
 
     def subject
-      if @opts[:use_site_subject]
+      if @opts[:template] &&
+          TranslationOverride.exists?(locale: I18n.locale, translation_key: "#{@opts[:template]}.subject_template")
+        subject = I18n.t("#{@opts[:template]}.subject_template", @template_args)
+      elsif @opts[:use_site_subject]
         subject = String.new(SiteSetting.email_subject)
         subject.gsub!("%{site_name}", @template_args[:email_prefix])
         subject.gsub!("%{optional_re}", @opts[:add_re_to_subject] ? I18n.t('subject_re') : '')

spec/components/email/message_builder_spec.rb

@@ -257,6 +257,20 @@ describe Email::MessageBuilder do
       expect(templated_builder.subject).to eq(rendered_template)
     end
 
+    context "when use_site_subject is true" do
+      let(:templated_builder) { Email::MessageBuilder.new(to_address, template: 'mystery', use_site_subject: true) }
+
+      it "can use subject override" do
+        override = TranslationOverride.create(
+          locale: I18n.locale,
+          translation_key: "mystery.subject_template",
+          value: "my customized subject"
+        )
+        I18n.expects(:t).with("mystery.subject_template", templated_builder.template_args).returns(override.value)
+        expect(templated_builder.subject).to eq(override.value)
+      end
+    end
+
   end
 
   context "from field" do