This is the last piece of https://gitlab.com/gitlab-org/gitlab/-/issues/282443+ (preliminary FF rollout issue at https://gitlab.com/gitlab-org/gitlab/-/work_items/596742+) before we can change it to a `gitlab_com_derisk`. The feature is complete on the backend, but the rich-text editor currently doesn't so much as recognise these embeds. We can probably get an MVC by treating them like videos, referring to the allowlist (and transform configuration) to know which ones are actually going to be treated as embeds by the backend.