Fix inline panel ordering

[lb-]

• Fixes Front-end reordering issue of orderables in Wagtail 4.0 & 4.0.1  #9145
• Re-ordering was no longer working with the new DOM structure introduced by Update InlinePanel and StreamField styles for new designs #8983 / ca7f23d
• Instead of using $.children use $.find to find the panel's input fields but ensure we still supported nested fields by finding based on the child panel's id
• Note: This is intentionally two discrete commits, the first being the smallest code change to get this fix in (for cherry picking to 4.0 stable), the second commit is a start on a bit more robust unit tests and moving the code to an area that has better linting by default.
• If merged we can create a new issue for enhancing these unit tests to cover the re-ordering bug and other basic behaviour.

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• Do the tests still pass?
• Does the code comply with the style guide?
• For front-end changes: Did you test on all of Wagtail’s supported environments? Chrome, Firefox, Safari all desktop on macOS

[squash-labs]

Manage this branch in Squash

Test this branch here: https://lb-fix9145-inline-panel-orderi-sj54p.squash.io

[gasman]

The logic here looks good to me - thanks for picking this up so quickly @lb-!

Just suggesting a small tweak out of paranoia over clashing child IDs (which is probably a reasonable default stance to take when it comes to this code :-) )

[gasman]

Merged in 174488a (main) and a5c1334 (stable/4.0.x - first commit only).

[lb-]

Thanks @gasman