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.

app/mailers/user_notifications.rb

@@ -12,6 +12,7 @@ class UserNotifications < ActionMailer::Base
   include ApplicationHelper
   helper :application, :email
   default charset: 'UTF-8'
+  layout 'email_template'
 
   include Email::BuildEmailHelper
 
@@ -362,11 +363,6 @@ class UserNotifications < ActionMailer::Base
     result
   end
 
-  class UserNotificationRenderer < ActionView::Base
-    include UserNotificationsHelper
-    include EmailHelper
-  end
-
   def self.get_context_posts(post, topic_user, user)
     if (user.user_option.email_previous_replies == UserOption.previous_replies_type[:never]) ||
        SiteSetting.private_email?
@@ -580,15 +576,7 @@ class UserNotifications < ActionMailer::Base
         site_description: SiteSetting.site_description
       )
 
-      unless translation_override_exists
-        html = UserNotificationRenderer.with_view_paths(Rails.configuration.paths["app/views"]).render(
-          template: 'email/invite',
-          format: :html,
-          locals: { message: PrettyText.cook(message, sanitize: false).html_safe,
-                    classes: Rtl.new(user).css_class
-          }
-        )
-      end
+      html = PrettyText.cook(message, sanitize: false).html_safe
     else
       reached_limit = SiteSetting.max_emails_per_day_per_user > 0
       reached_limit &&= (EmailLog.where(user_id: user.id)
@@ -608,7 +596,6 @@ class UserNotifications < ActionMailer::Base
       end
 
       unless translation_override_exists
-
         html = UserNotificationRenderer.with_view_paths(Rails.configuration.paths["app/views"]).render(
           template: 'email/notification',
           format: :html,
@@ -651,7 +638,6 @@ class UserNotifications < ActionMailer::Base
       site_description: SiteSetting.site_description,
       site_title: SiteSetting.title,
       site_title_url_encoded: URI.encode(SiteSetting.title),
-      style: :notification,
       locale: locale
     }
 
@@ -689,13 +675,6 @@ class UserNotifications < ActionMailer::Base
     @anchor_color    = ColorScheme.hex_for_name('tertiary')
     @markdown_linker = MarkdownLinker.new(@base_url)
     @unsubscribe_key = UnsubscribeKey.create_key_for(@user, "digest")
-  end
-
-  def apply_notification_styles(email)
-    email.html_part.body = Email::Styles.new(email.html_part.body.to_s).tap do |styles|
-      styles.format_basic
-      styles.format_notification
-    end.to_html
-    email
+    @disable_email_custom_styles = !SiteSetting.apply_custom_styles_to_digest
   end
 end