GitLab

The TutorialURL will be opened in a sidebar in MIT App Inventor. This
permits people to build “template” projects with a tutorial (hosted on
their own site) displayed in a sidebar.

For now, we only allow tutorial urls from http://appinventor.mit.edu/ or
http://appinv.us/.

Change-Id: Ia79eeac6bd3809ce32d0475aa3954ad2c996c776