core: add sanitization of text/html-video-embed. Move #context handling out of z_html to z_sanitize.

(cherry picked from commit edd1ceff22627480e596dccdc4145acb1f6ec8ce)
core: add acl mime check to m_media:replace/3.

(cherry picked from commit 523e55124c4e20054ed7a99868441e87150ffa0f)
mod_admin: when replacing a media item, show the oembed/video-embed panels for embedded content.

(cherry picked from commit 5ae977460d74368b09588a641d0334576f262423)