Use PajamasComponent in admin nav header

The following discussion from !115162 (merged) should be addressed once the merge request is merged:

@ohoral started a discussion: (+3 comments)

nitpick: I can't figure out when this is rendered but it probably should use the Pajamas Avatar Component

Or at least the s32 container CSS class is not suitable for the 18px large icon (and by the way we do not support 18px-sized icons).

In app/views/shared/nav/_admin_scope_header.html.haml, we need to use the PajamasAvatar component.