Drawer component to eventually render "Whats New" content.

This is one of many commits/MR's that will eventually be the entire
feature. The drawer is hidden behind the :whats_new_drawer feature flag.

part of:
https://gitlab.com/groups/gitlab-org/-/epics/3556
https://gitlab.com/gitlab-org/growth/engineering/-/issues/5387
https://gitlab.com/gitlab-org/growth/engineering/-/issues/5388