The `get_course_last_published` function is used by CourseGraph to
determine whether or not a course should be dumped to Neo4j.
If the course hasn't been published since it was last dumped to
Neo4j, then it can be skipped (unless the override_cache option
is enabled).
The function was previously built using the BlockStructure
data model. While this worked fine in Production instances that
enable `block_structure.storage_backing_for_cache`, this
implementation did NOT work in development environments,
which do not use the BlockStrcture model.
Instead, we switch to using CourseOverview.modified to
approximate when a course was last published. This is method
has fewer moving parts and is universally available across
instances.
d75a32c009