Archiving and restoring blocks can cause duplicate sort values

[ScopeyNZ]

Currently when you archive a block, it's sort order is kept. If you then restore a block a bit later, after the set has been modified/re-arranged, then it's likely you'll have a conflict with the sort order number.

We should either;

• Clear the existing sort order when elements are archived, and ensure the sort order is reset when restoring the block (like we do in BaseElement::onBeforeWrite already), or
• Re-calculate the sort order when we restore a block