Propose a revamp of the global administration UI

Goals

As this UI is one of the first screens any new Admin sees in XWiki it’s important that we:

• improve feature discoverability and understanding
• enhance consistency
• improve aesthetics

Design page: https://design.xwiki.org/xwiki/bin/view/Proposal/GlobalAdminUIrevamp
Forum discussion: https://forum.xwiki.org/t/global-administration-ui-revamp/13986