Update FormDocumentSyncService for new unique indexes

[stephencdaly]

What problem does this pull request solve?

Trello card: https://trello.com/c/dGv0TrD1

We're not yet updating the service to increment the version number when a form goes live.

However, we need to update the service to handle the new unique indexes we want to add that will enforce that there is only one form document per version per language for a form.

Ensure we delete archived form documents before creating a new live form document so we only ever have one live or archived form document per language per form.

We also don't need to handle the case where there is an existing archived form document when archiving a form, as this can never happen and will soon be enforced by the new unique indexes.

I've verified that no existing form has both live and archived form documents for the same language on any environment using the query:

SELECT form_id from form_documents where tag IN ('live', 'archived') group by form_id, language having count(1) > 1;

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