Previously, CMS Django admin would demand that at least one
LTIConfig be associated with the new library. This is not
necessary, but Django thought it was, because the `blank`
flag was not enabled on the ContentLibrary model's
`authored_lti_config` many-to-many field.

This should not affect the experimental blockstore-based
library authoring frontend, which already allows libraries
to be created without any authorized_lti_configs.

This will not affect the existing modulestore-based
library authoring frontend, since it doesn't even use
any of the models in question.

A migration is included with this commit, but it should NOT
change MySQL schema, since `blank` is a Django-level validation
flag.