[self-hosted] UI: Re-instantiate first-steps/initial-setup page

This is the first page you see when you visit https://your-gitpod-installation.com after installing it. It needs to accomplish two things:
 - get the installers consent on the terms
 - guide them to properly configure a Git Hoster (GitHub/Gitlab)

Fixes:
 - https://github.com/gitpod-io/gitpod/issues/3904
 - #3705

/cc @gtsiolis @jankeromnes 