Add admin page for targeted messages basic upload

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183052



Merged-by: Ian Anderson <ianderson@gitlab.com>
Approved-by: Minahil Nichols <minahilnichols@gitlab.com>
Approved-by: Ian Anderson <ianderson@gitlab.com>
Reviewed-by: Ian Anderson <ianderson@gitlab.com>
Reviewed-by: Alex Pennells <apennells@gitlab.com>
Co-authored-by: rliu-gl <rliu@gitlab.com>