feat: use gravatar if no avatar image (#1307)

Fixes:  #1240

app/models/user.rb

@@ -110,6 +110,15 @@ class User < ApplicationRecord
     inboxes.where(account_id: Current.account.id)
   end
 
+  alias avatar_img_url avatar_url
+  def avatar_url
+    if avatar_img_url == ''
+      hash = Digest::MD5.hexdigest(email)
+      return "https://www.gravatar.com/avatar/#{hash}"
+    end
+    avatar_img_url
+  end
+
   def administrator?
     current_account_user&.administrator?
   end