CustomButtonSet - don't touch children/members directly, set button_order

[himdel]

Depends on ManageIQ/manageiq#18368

The idea is, we shold not try to manipulate both button_order and members for CustomButtons,
members should always follow button_order.

The core PR adds an after_save to CustomButtonSet that always updates the set of children to match button_order.

This PR removes all code which updates members/children, and makes sure button_order gets updated instead.

---

This probably needs some testing,
I'm not quite sure what..

      org_mems = @custom_button_set.members # clean up existing members
      org_mems.each do |m|
        uri = CustomButton.find(m.id)
        uri.save
      end

(used twice)

was supposed to achieve, find+save sound like a no-op.

[himdel]

Cc @martinpovolny

[himdel]

funnily enough, comparing integers and CustomButton instances probably didn't work

[miq-bot]

Checked commit https://github.com/himdel/manageiq-ui-classic/commit/d7a7948bfff3ee5df5df3b26282a8008c7ac6884 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 0 offenses detected
Everything looks fine. 🍰

[himdel]

ManageIQ/manageiq#18368 went in hammer after all,
so adding hammer/yes

https://bugzilla.redhat.com/show_bug.cgi?id=1770300 (not a full fix though)