Fix GitLab project import when a user has access only to their default namespace.

Render a hidden field with namespace ID so it can be read by JavaScript
and passed to "/import/gitlab_project/new" screen.

app/views/projects/new.html.haml

@@ -27,6 +27,7 @@
                 - else
                   .input-group-addon.static-namespace
                     #{root_url}#{current_user.username}/
+                  = f.hidden_field :namespace_id, value: current_user.namespace_id
           .form-group.col-xs-12.col-sm-6.project-path
             = f.label :namespace_id, class: 'label-light' do
               %span