Allow embedding (YouTube) videos on custom pages (and possibly the event description)

For conferences this may be quite useful if there is e.g. a teaser video about the conference.

Whitelisting `` is probably not the best idea, but maybe we could find some other way to do it..

Start by tracing how custom pages and event descriptions are rendered and sanitized. Review the existing handling of embedded HTML and determine a safe, explicit video-embedding scope for both locations; done means the supported video content renders without permitting unsafe embeds, with coverage for the affected entry points.

Written by the indexing model from the issue text.