Filter out img, video and audio tags in notice HTML

Filter out img, video and audio tags in notice HTML

Because we don't want to auto-fetch items from a remote server. Such
items should be delivered as attachment metadata and portrayed in the
way the local instance chooses.

Choices for portrayal are either simply nullifying this and embedding
the data, linking the file remotely requiring a manual click or maybe
use remote oEmbed data etc. to download files locally so no remote
requests have to be made.