Fixed nested plugins

Plugins inside the text plugin were not being correctly associated with its parent because it was still submitting 'parent_id' to the backend but django-cms deprecated and removed this in https://github.com/divio/django-cms/commit/f4817756ecfc8e61f32067cb5345d03ae4ec6ecd and is now expected to use 'plugin_parent' as the POST param key.

See https://github.com/divio/django-cms/issues/3562 for detailed description of bug.