Check redirect path in the continue_params

Fixes
https://dev.gitlab.org/gitlab/gitlabhq/issues/2649
https://gitlab.com/gitlab-org/gitlab-ce/issues/13956

app/controllers/concerns/continue_to_params.rb

+module ContinueToParams
+  extend ActiveSupport::Concern
+
+  def continue_params
+    continue_params = params[:continue]
+    return nil unless continue_params
+
+    continue_params       = continue_params.permit(:to, :notice, :notice_now)
+    continue_params[:to]  = root_url unless continue_params[:to].start_with?('/')
+
+    continue_params
+  end
+end

app/controllers/projects/forks_controller.rb

 class Projects::ForksController < Projects::ApplicationController
+  include ContinueToParams
+
   # Authorize
   before_action :require_non_empty_project
   before_action :authorize_download_code!
@@ -53,15 +55,4 @@ class Projects::ForksController < Projects::ApplicationController
       render :error
     end
   end
-
-  private
-
-  def continue_params
-    continue_params = params[:continue]
-    if continue_params
-      continue_params.permit(:to, :notice, :notice_now)
-    else
-      nil
-    end
-  end
 end

app/controllers/projects/imports_controller.rb

 class Projects::ImportsController < Projects::ApplicationController
+  include ContinueToParams
+
   # Authorize
   before_action :authorize_admin_project!
   before_action :require_no_repo, only: [:new, :create]
@@ -44,16 +46,6 @@ class Projects::ImportsController < Projects::ApplicationController
 
   private
 
-  def continue_params
-    continue_params = params[:continue]
-
-    if continue_params
-      continue_params.permit(:to, :notice, :notice_now)
-    else
-      nil
-    end
-  end
-
   def finished_notice
     if @project.forked?
       'The project was successfully forked.'