Forum: Add the ability to lock discussions manually

Test on any device. Log in a 3.7 site as a teacher. Go to a forum and add a new discussion. Go to the created discussion and open the context menu. Check that the menu contains an item called "Lock this discussion" with a closed lock icon. Click the "Lock this discussion" item. Check this message is displayed on top of the discussion: "This discussion has been locked so you can no longer reply to it." Check that you can reply to the discussion (this is inconsistent with the message but it's the same in web). Check that the menu item is replaced with one called "Unlock this discussion" and the icon is now an open lock. Log in the same site as a student and go to the forum discussion. Check this message is displayed on top of the discussion: "This discussion has been locked so you can no longer reply to it." Check that you cannot reply to the discussion. Log in the site again as a teacher and go to the forum discussion. Click "Unlock this discussion" in the context menu. Check that the message on top of the discussion is no longer displayed. Log in the same site as a student and go to the forum discussion. Check that you can now reply to the discussion. Log in a 3.6 site as a teacher. Go to a forum discussion. Check the context menu has no item to lock or unlock the discussion.