review all the core .schema files

[moloko]

There's a few entries in course.model.schema that don't look to be used any more and I'm sure there will be others.

Similarly there's some properties that are missing (e.g. _isVisible) and some discrepancies between the versions in the framework and those in the AT - for example contentobject.model.schema has pageBody in the framework version but not in the authoring tool version.

[moloko]

Ones we've found so far that don't seem to be needed any more:

• _ariaLabels.menu
• _ariaLabels.menuItem
• _ariaLabels.menuViewButton
• _ariaLabels.navigationBack
• _ariaLabels.closeResources
• _ariaLabels.drawerBack (_ariaLabels.previous is what's currently assigned to the Drawer's back button in drawer.hbs)
• _ariaLabels.menuBack

[moloko]

instruction seems to be missing for both article and block - it's referenced in article.hbs and block.hbs but not in the .schema files

[moloko]

AT develop branch has had _onScreen added to contentObject/article/block/component schemas, need to add this to the framework's files