Studio support for creating and editing libraries (PR 6046)

Duplicates https://github.com/edx/edx-platform/pull/6046 to allow Marco to review without spamming the reviewers on the original PR.
The error message was copied from the courses (https://github.com/edx/edx-platform/blob/16809ddf1c32b04dc892e177b486bc63344e4824/cms/djangoapps/contentstore/views/course.py#L576-L579). If we change it for the libraries it would probably make sense to change it for courses, too.