Only allow embeds on media_domain.

This prevents leaking of modhashes etc. from logged-in embeds.
WebSocket: Add client-side support for new-style update messages.

Step one of a live migration to a new format for update messages.
WebSocket: start sending new-style update messages.

These include more metadata rather than just the pre-rendered update.
WebSocket: remove compatibility code for old-style update messages.
Add support for optional desktop notifications of new updates.