Fix missing topic link when private email enabled (#39667)

#### Description 

Currently, staged users with `private_email enabled` receive emails
without any content or link to the topic the conversation is taking
place on, making it impossible to navigate to it. This PR adds the
response instruction section, which contains the `topic_url` when this
happens, so staged users can visit the topic.

meta:
[/t/401078](https://meta.discourse.org/t/with-private-email-enabled-we-should-still-link-the-topic-in-outgoing-emails/401078)

---------

Co-authored-by: Bannon Tanner <bannon@discourse.org>

app/mailers/user_notifications.rb

@@ -750,9 +750,7 @@ class UserNotifications < ActionMailer::Base
       end
 
       first_footer_classes = "highlight"
-      if (allow_reply_by_email && user.staged) || (user.suspended? || user.staged?)
-        first_footer_classes = ""
-      end
+      first_footer_classes = "" if user.suspended? || (user.staged? && !SiteSetting.private_email?)
 
       unless translation_override_exists
         html =
@@ -786,7 +784,7 @@ class UserNotifications < ActionMailer::Base
       mailing_list_mode: user.user_option.mailing_list_mode,
       unsubscribe_url: post.unsubscribe_url(user),
       allow_reply_by_email: allow_reply_by_email,
-      only_reply_by_email: allow_reply_by_email && user.staged,
+      only_reply_by_email: allow_reply_by_email && user.staged? && !SiteSetting.private_email?,
       use_site_subject: use_site_subject,
       add_re_to_subject: add_re_to_subject,
       show_category_in_subject: show_category_in_subject,
@@ -794,7 +792,8 @@ class UserNotifications < ActionMailer::Base
       private_reply: post.topic.private_message?,
       subject_pm: subject_pm,
       participants: participants,
-      include_respond_instructions: !(user.suspended? || user.staged?),
+      include_respond_instructions:
+        !(user.suspended? || (user.staged? && !SiteSetting.private_email?)),
       notification_type: notification_type,
       template: template,
       use_topic_title_subject: use_topic_title_subject,

config/locales/server.en.yml

@@ -4821,6 +4821,8 @@ en:
 
         %{message}
 
+        %{respond_instructions}
+
     account_suspended:
       title: "Account Suspended"
       subject_template: "[%{email_prefix}] Your account has been suspended"

spec/mailers/user_notifications_spec.rb

@@ -1158,7 +1158,7 @@ RSpec.describe UserNotifications do
 
   shared_examples "respect for private_email" do
     context "with private_email" do
-      it "doesn't support reply by email" do
+      it "doesn't include topic title or slug for regular users" do
         SiteSetting.private_email = true
 
         mailer =
@@ -1177,6 +1177,35 @@ RSpec.describe UserNotifications do
         expect(message.text_part.body.to_s).not_to include(topic.title)
         expect(message.text_part.body.to_s).not_to include(topic.slug)
       end
+
+      it "still links back to the topic for staged users" do
+        skip_types = %i[linked quoted mentioned group_mentioned]
+        if skip_types.include?(notification_type)
+          skip "Staged users don't receive #{notification_type} emails"
+        end
+
+        invite_types = %i[invited_to_private_message invited_to_topic watching_first_post]
+        if invite_types.include?(notification_type)
+          skip "Invite-type emails use a different template structure"
+        end
+
+        SiteSetting.private_email = true
+        user.update!(staged: true)
+
+        mailer =
+          UserNotifications.public_send(
+            mail_type,
+            user,
+            notification_type: Notification.types[notification.notification_type],
+            notification_data_hash: notification.data_hash,
+            post: notification.post,
+          )
+        message = mailer.message
+
+        slugless_path = notification.post.topic.slugless_url
+        expect(message.html_part.body.to_s).to include(slugless_path)
+        expect(message.text_part.body.to_s).to include(slugless_path)
+      end
     end
   end