mgr/dashboard: Add configurable MOTD or wall notification

mgr/dashboard: Add configurable MOTD or wall notification

Fixes: https://tracker.ceph.com/issues/51408
Signed-off-by: Volker Theile <vtheile@suse.com>
(cherry picked from commit f7f163e75cf5fb6cd022a8d13c28f5b923e01aed)

Conflicts:
	src/pybind/mgr/dashboard/frontend/src/app/core/navigation/navigation/navigation.component.ts
    src/pybind/mgr/dashboard/frontend/src/app/shared/api/motd.service.spec.ts
	src/pybind/mgr/dashboard/frontend/src/app/shared/components/alert-panel/alert-panel.component.html
	src/pybind/mgr/dashboard/frontend/src/app/shared/components/alert-panel/alert-panel.component.ts
	src/pybind/mgr/dashboard/frontend/src/app/shared/components/components.module.ts
    src/pybind/mgr/dashboard/frontend/src/app/shared/pipes/sanitize-html.pipe.spec.ts
    src/pybind/mgr/dashboard/frontend/src/app/shared/services/motd-notification.service.ts
    src/pybind/mgr/dashboard/frontend/src/app/shared/services/motd-notification.service.spec.ts
	src/pybind/mgr/dashboard/module.py
    src/pybind/mgr/dashboard/plugins/motd.py
	src/python-common/tox.ini
    src/python-common/ceph/utils.py