Ability to customize the logo in digest emails

app/helpers/user_notifications_helper.rb

@@ -26,7 +26,8 @@ module UserNotificationsHelper
   end
 
   def logo_url
-    logo_url = SiteSetting.logo_url
+    logo_url = SiteSetting.digest_logo_url
+    logo_url = SiteSetting.logo_url if logo_url.blank?
     if logo_url !~ /http(s)?\:\/\//
       logo_url = "#{Discourse.base_url}#{logo_url}"
     end

config/locales/server.en.yml

@@ -610,6 +610,7 @@ en:
     onebox_max_chars: "Maximum characters a onebox will import from an external website into the post"
 
     logo_url: "The logo for your site eg: http://example.com/logo.png"
+    digest_logo_url: "The logo used in your site's email digest. If left blank `logo_url` will be used. eg: http://example.com/logo.png"
     logo_small_url: "The small logo for your site used when scrolling down on topics eg: http://example.com/logo-small.png"
     favicon_url: "A favicon for your site, see http://en.wikipedia.org/wiki/Favicon"
     apple_touch_icon_url: "Icon used for Apple touch devices. Recommended size is 144px by 144px."

config/site_settings.yml

@@ -12,6 +12,9 @@ required:
   logo_small_url:
     client: true
     default: '/images/d-logo-sketch-small.png'
+  digest_logo_url:
+    client: false
+    default: ''
   favicon_url:
     client: true
     default: '/images/default-favicon.ico'