Fix sortable elements

[tvdeyen]

What is this pull request for?

1. Fix sorting of nested elements of freshly created elements Make sure that nested elements of all elements get initialized as sortable after being created, not just fixed.

2. Avoid sending sort events twice SortableJS sends two events if you drag an element from one nesteable elements container to another. Leading to incrementing the target lists positions to high.

Since acts_as_list maintains both lists correctly we can simply suppress the second event on the source list.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message

[alchemycms-bot]

💚 All backports created successfully

Status	Branch	Result
✅	7.1-stable

Questions ?

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