NcModal close button is inaccessible with scrollable content

[Antreesy]

Issue:

• Probably a regression from fix(NcModal): Close button should be visible even if modal content is scrolled #4350
• After re-ordering of DOM elements modal-container__close and modal-container__content content can overlap close button, making it inaccessible

Screenshots:

Possible solution:

• set z-index: 1 for modal-container__close

Originally posted by @Antreesy in #4350 (comment)

[susnux]

BTW this is caused by talk resetting the modal content margin, normally there is a margin of 50px so there should be not content overlapping the button.