WIP: Form transition : legacy (full page) or modal

[btry]

A new option to choose how to show a form

Not mergeable because of a bug between select2 dropdowns and modals, probably introduced by https://github.com/glpi-project/glpi/pull/9533

Ig a select2 dropdown is located far enough to require the user to scroll, then clicking the select2 will show the items in a mispalced position : it is not above or under the select2.