FEATURE: customization of html emails (#7934)

This feature adds the ability to customize the HTML part of all emails using a custom HTML template and optionally some CSS to style it. The CSS will be parsed and converted into inline styles because CSS is poorly supported by email clients. When writing the custom HTML and CSS, be aware of what email clients support. Keep customizations very simple.

Customizations can be added and edited in Admin > Customize > Email Style.

Since the summary email is already heavily styled, there is a setting to disable custom styles for summary emails called "apply custom styles to digest" found in Admin > Settings > Email.

As part of this work, RTL locales are now rendered correctly for all emails.

spec/mailers/user_notifications_spec.rb

@@ -260,7 +260,7 @@ describe UserNotifications do
       expect(mail.subject).to match(/Taggo/)
       expect(mail.subject).to match(/Taggie/)
 
-      mail_html = mail.html_part.to_s
+      mail_html = mail.html_part.body.to_s
 
       expect(mail_html.scan(/My super duper cool topic/).count).to eq(1)
       expect(mail_html.scan(/In Reply To/).count).to eq(1)
@@ -287,7 +287,7 @@ describe UserNotifications do
         notification_data_hash: notification.data_hash
       )
 
-      expect(mail.html_part.to_s.scan(/In Reply To/).count).to eq(0)
+      expect(mail.html_part.body.to_s.scan(/In Reply To/).count).to eq(0)
 
       SiteSetting.enable_names = true
       SiteSetting.display_name_on_posts = true
@@ -304,7 +304,7 @@ describe UserNotifications do
         notification_data_hash: notification.data_hash
       )
 
-      mail_html = mail.html_part.to_s
+      mail_html = mail.html_part.body.to_s
       expect(mail_html.scan(/>Bob Marley/).count).to eq(1)
       expect(mail_html.scan(/>bobmarley/).count).to eq(0)
 
@@ -317,7 +317,7 @@ describe UserNotifications do
         notification_data_hash: notification.data_hash
       )
 
-      mail_html = mail.html_part.to_s
+      mail_html = mail.html_part.body.to_s
       expect(mail_html.scan(/>Bob Marley/).count).to eq(0)
       expect(mail_html.scan(/>bobmarley/).count).to eq(1)
     end
@@ -331,8 +331,8 @@ describe UserNotifications do
         notification_data_hash: notification.data_hash
       )
 
-      expect(mail.html_part.to_s).to_not include(response.raw)
-      expect(mail.html_part.to_s).to_not include(topic.url)
+      expect(mail.html_part.body.to_s).to_not include(response.raw)
+      expect(mail.html_part.body.to_s).to_not include(topic.url)
       expect(mail.text_part.to_s).to_not include(response.raw)
       expect(mail.text_part.to_s).to_not include(topic.url)
     end
@@ -365,10 +365,10 @@ describe UserNotifications do
       expect(mail.subject).not_to match(/Uncategorized/)
 
       # 1 respond to links as no context by default
-      expect(mail.html_part.to_s.scan(/to respond/).count).to eq(1)
+      expect(mail.html_part.body.to_s.scan(/to respond/).count).to eq(1)
 
       # 1 unsubscribe link
-      expect(mail.html_part.to_s.scan(/To unsubscribe/).count).to eq(1)
+      expect(mail.html_part.body.to_s.scan(/To unsubscribe/).count).to eq(1)
 
       # side effect, topic user is updated with post number
       tu = TopicUser.get(post.topic_id, user)
@@ -384,7 +384,7 @@ describe UserNotifications do
         notification_data_hash: notification.data_hash
       )
 
-      expect(mail.html_part.to_s).to_not include(response.raw)
+      expect(mail.html_part.body.to_s).to_not include(response.raw)
       expect(mail.text_part.to_s).to_not include(response.raw)
     end
 
@@ -451,13 +451,13 @@ describe UserNotifications do
       expect(mail.subject).to include("[PM] ")
 
       # 1 "visit message" link
-      expect(mail.html_part.to_s.scan(/Visit Message/).count).to eq(1)
+      expect(mail.html_part.body.to_s.scan(/Visit Message/).count).to eq(1)
 
       # 1 respond to link
-      expect(mail.html_part.to_s.scan(/to respond/).count).to eq(1)
+      expect(mail.html_part.body.to_s.scan(/to respond/).count).to eq(1)
 
       # 1 unsubscribe link
-      expect(mail.html_part.to_s.scan(/To unsubscribe/).count).to eq(1)
+      expect(mail.html_part.body.to_s.scan(/To unsubscribe/).count).to eq(1)
 
       # side effect, topic user is updated with post number
       tu = TopicUser.get(topic.id, user)
@@ -473,8 +473,8 @@ describe UserNotifications do
         notification_data_hash: notification.data_hash
       )
 
-      expect(mail.html_part.to_s).to_not include(response.raw)
-      expect(mail.html_part.to_s).to_not include(topic.url)
+      expect(mail.html_part.body.to_s).to_not include(response.raw)
+      expect(mail.html_part.body.to_s).to_not include(topic.url)
       expect(mail.text_part.to_s).to_not include(response.raw)
       expect(mail.text_part.to_s).to_not include(topic.url)
     end
@@ -635,7 +635,7 @@ describe UserNotifications do
 
     # WARNING: you reached the limit of 100 email notifications per day. Further emails will be suppressed.
     # Consider watching less topics or disabling mailing list mode.
-    expect(mail.html_part.to_s).to match(I18n.t("user_notifications.reached_limit", count: 2))
+    expect(mail.html_part.body.to_s).to match(I18n.t("user_notifications.reached_limit", count: 2))
     expect(mail.body.to_s).to match(I18n.t("user_notifications.reached_limit", count: 2))
   end