[iOS] Make inactive posting buttons/fields for "posting disabled" courses

[sergeymomot]

In some courses (with "Archived" status) posting in Discussions is disabled ("Posting in discussions is disabled by the course team"). On the Web and in edX app all make Post/Response/Comment options are hidden or inactive. We also need to disable (or hide) the "Create new post" button and the "Add a response" / "Add a comment" fields for these courses. Course example