user invites on-prem: Initial front-end

Created by: philipp-spiess

Part of https://github.com/sourcegraph/sourcegraph/issues/31214

After spending time exploring different options, we have agreed upon a way of adding the invitations features on the home page for both cloud and on-prem in https://www.figma.com/proto/Og1zVk7BbZ7SWTXM5WsWA5/Account-Setups-OKR-explorations?page-id=28%3A8126&node-id=29%3A12764&scaling=min-zoom&starting-point-node-id=29%3A12764

This task is about the Front-End changes to the home page to enable that.