Resolve "Icons missing on oauth pages"

What does this MR do and why?

Resolves #414418 (closed). Fixes the missing icons on OAuth page in new navigation.

Icons in the new nav are now rendered with Vue and need gon.sprite_icons which was not provided in this controller.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Check locally on http://gdk.test:3000/oauth/authorize
• It will complain about a missing client_id. Ignore that as it's unrelated to the icons.
• See the icons in the new sidebar.

MR acceptance checklist

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