Show discourse logo as system user avatar

app/models/user.rb

@@ -340,7 +340,7 @@ class User < ActiveRecord::Base
   end
 
   def avatar_template
-    uploaded_avatar_path || User.gravatar_template(email)
+    uploaded_avatar_path || User.gravatar_template(id != -1 ? email : "team@discourse.org")
   end
 
   # The following count methods are somewhat slow - definitely don't use them in a loop.

app/serializers/user_action_serializer.rb

@@ -30,6 +30,7 @@ class UserActionSerializer < ApplicationSerializer
 
   def avatar_template
     avatar_for(
+      object.user_id,
       object.email,
       object.use_uploaded_avatar,
       object.uploaded_avatar_template,
@@ -39,6 +40,7 @@ class UserActionSerializer < ApplicationSerializer
 
   def acting_avatar_template
     avatar_for(
+      object.acting_user_id,
       object.acting_email,
       object.acting_use_uploaded_avatar,
       object.acting_uploaded_avatar_template,
@@ -72,9 +74,10 @@ class UserActionSerializer < ApplicationSerializer
 
   private
 
-  def avatar_for(email, use_uploaded_avatar, uploaded_avatar_template, uploaded_avatar_id)
+  def avatar_for(user_id, email, use_uploaded_avatar, uploaded_avatar_template, uploaded_avatar_id)
     # NOTE: id is required for cases where the template is blank (during initial population)
     User.new(
+      id: user_id,
       email: email,
       use_uploaded_avatar: use_uploaded_avatar,
       uploaded_avatar_template: uploaded_avatar_template,