fix: update modulestore migrator to not publish in draft context

[bradenmacdonald]

Description

This fixes a few issues:

Issue 1: importing a course or legacy library using modulestore_migrator currently creates a single draft changelog, but many separate publish logs - one for each component or container. It should only create a single publish log per import.

Issue 2: modulestore_migrator is doing a weird call to save() the draft change log, which doesn't seem to make any sense, and is expressly forbidden by openedx-core's API conventions.

Issue 3: We have considered changing openedx-core so that it doesn't allow publishing entities within a "bulk draft changes" context, which would help ensure the integrity of both our draft and published change logs / data. However, modulestore_migrator currently publishes things within a bulk draft changes context.

Testing instructions

Needs some careful thought to know if publishing the entities in this different way is going to cause any issues.

Deadline

None

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