Use TransferContext for project

app/contexts/projects/transfer_context.rb

+module Projects
+  class TransferContext < BaseContext
+    def execute(role = :default)
+      namespace_id = params[:project].delete(:namespace_id)
+      allowed_transfer = can?(current_user, :change_namespace, project) || role == :admin
+
+      if allowed_transfer && namespace_id.present?
+        if namespace_id == Namespace.global_id
+          if project.namespace.present?
+            # Transfer to global namespace from anyone
+            project.transfer(nil)
+          end
+        elsif namespace_id.to_i != project.namespace_id
+          # Transfer to someone namespace
+          namespace = Namespace.find(namespace_id)
+          project.transfer(namespace)
+        end
+      end
+
+    rescue ProjectTransferService::TransferError => ex
+      project.reload
+      project.errors.add(:namespace_id, ex.message)
+      false
+    end
+  end
+end
+

app/views/projects/transfer.js.haml

+- if @project.errors[:namespace_id].present?
+  :plain
+    $("#tab-transfer .errors-holder").replaceWith(errorMessage('#{escape_javascript(@project.errors[:namespace_id].first)}'));
+    $("#tab-transfer .form-actions input").removeAttr('disabled').removeClass('disabled');
+- else
+  :plain
+    location.href = "#{edit_project_path(@project)}";