minor #1936 [Site] Use ux-icons (1/2) (smnandre)

This PR was squashed before being merged into the 2.x branch.

Discussion
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[Site] Use ux-icons (1/2)

First pass to switch from the old custom Icon component to ux-icons

Next step (once feature available on ux-icons)
* configure class & data-attributes auto-attributes
* use <twig:ux:icon instead of <twig:Icon

Commits
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314723c [Site] Use ux-icons (1/2)

Author: kbond