Prior to this commit, any apps that update their data when courses are
published had to create their own management commands for bootstrapping
or error recovery (e.g. generate_course_overviews,
generate_course_blocks). This is a management command to allow us to
generically simulate a course_publish signal so that any async tasks
that trigger actions off of that can do so without having to each write
their own management commands.
It has a few options to make it more ops friendly:
* Can specify a set of courses, but defaults to all courses in the
modulestore.
* Can specify a set of listeners, so we can bootstrap a new app without
rebuilding everything.
* Can specify a delay between emitting signals so that we don't flood
the queues and block author-initiated publishes from going through in
a timely manner.
* Dry-run mode for a simple preview of what the script will attempt.
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