Merge branch '454964-user-mapping-fe-add-basic-app-structure-and-components-ff' into 'master'

Add basic PlaceholdersTabApp and feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149735



Merged-by: Mireya Andres <mandres@gitlab.com>
Approved-by: Marc Saleiko <msaleiko@gitlab.com>
Approved-by: Bala Kumar <sbalakumar@gitlab.com>
Approved-by: James Rushford <jrushford@gitlab.com>
Approved-by: Mireya Andres <mandres@gitlab.com>
Reviewed-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Reviewed-by: Mireya Andres <mandres@gitlab.com>
Reviewed-by: Marc Saleiko <msaleiko@gitlab.com>
Co-authored-by: Justin Ho Tuan Duong <hduong@gitlab.com>