Adds the concept of "downstream-only" fields to the XBlock upstream sync logic.
Downstream-only fields are customizable fields set only on the downstream XBlock -- we don't keep track of the upstream field value anywhere on the downstream XBlock. Changes made to these fields in the upstream block are ignored, and if the link to the upstream block is severed, the downstream changes are preserved (not reset back to defaults, like the upstream-tracked customizable fields are).
The fields chosen as "downstream-only" are those related to scoring and grading.
The `max_attempts` field was previously a customizable field that tracked the upstream value. However, because it is scoring-related, it has been converted to a "downstream-only" field.
This change impacts course authors' use of library content in their courses.
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