Consider refactoring and splitting up sync.py

As noted in !7 (merged) the main sync.py script has become quite long and unwieldy. Consider refactoring it, splitting it up into smaller pieces. The main block comments indicate the various stages of the sync job, and might mark sensible places to split it up.

Related comment: !7 (comment 152859)

changed milestone to %Sprint 102

Penciling in for %Sprint 102 for sizing.

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mentioned in issue #29 (closed)

assigned to @rjg21

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Actually ended up doing this in preparation for adding deletion code #29 (closed)

Will probably have a MR for this before deletion as it will be big by itself.

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mentioned in merge request !19 (merged)

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marked this issue as related to #29 (closed)

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mentioned in commit 63883867

closed with merge request !19 (merged)