If course allows anonymous posts, and user is an author of a post, add
`anonymous` to posts' editable fields.
If course allows anonymous to peers posts, and user is an author of a
post, add `anonymous_to_peers` to posts' editable fields.
Course endpoint response to get request will now include course's
`allow_anonymous` and `allow_anonymous_to_peers` settings.
Added `anonymous` and `anonymous_to_peers` fields to the content
serializer, from which ThreadSerializer and CommentSerializer inherit.
Both fields have a default value of False, because there are cases where
course configuration doesn't allow them to be initialized (if a course
doesn't allow anonymous posts, the fields won't be included in the list
of initializable/editable fields).
eabd6e8019