Add API for getting need_reboot flag
We want to add ability to reboot instance from every page of admin-fe (admin-fe#98 (closed)). In order to render the reboot button we need an API that would return need_reboot flag. Now we get it when we fetch config (
api/pleroma/admin/config). I think it wouldn't be right to fetch settings on every page, so let's have a separate one.