Optimize Element.remove_child

Requires !1406 (merged)

While spending a lot of time updating all the SQL assertions, I noticed Element.remove_child made a lot of unnecessary queries and loaded unnecessary columns and models. I made some quick changes that should avoid loading child element UUIDs in RAM and avoid making one SQL query for each child element. There is definitely room for more improvement should we ever switch to full-SQL path manipulation methods, but this is a quick win.

assigned to @erwanrouchet

changed the description

requested review from @babadie

please rebase

added 5 commits

• 8524005f...8f2650f2 - 4 commits from branch master
• a7be6209 - Optimize Element.remove_child

Compare with previous version

mentioned in commit 32d7af3f

merged