Copy, don't move uploaded avatar files

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/26158

app/uploaders/avatar_uploader.rb

@@ -10,4 +10,15 @@ def store_dir
   def exists?
     model.avatar.file && model.avatar.file.exists?
   end
+
+  # We set move_to_store and move_to_cache to 'false' to prevent stealing
+  # the avatar file from a project when forking it.
+  # https://gitlab.com/gitlab-org/gitlab-ce/issues/26158
+  def move_to_store
+    false
+  end
+
+  def move_to_cache
+    false
+  end
 end