markup & classes needs to be updated for install-page.html.twig

Problem/motivation

install pages markup have the legacy of Drupal7 - needs to get its markup updated so it follow the same markup structure as starks page.html.twig

Proposed solution

update it so uses same page classes as #2011578: Markup for Stark's page.html.twig + maintenance-page.html.twig

TODO

as soon as #2011578: Markup for Stark's page.html.twig + maintenance-page.html.twig is rtbc it make sense to work on it