This simply wraps the field in extra text that the frontend will
deal with. Also added some helpers for extracting and stripping
audio and TTS tags from the rendered text.
This is paving the way to move the standard filters into Rust.
Non-empty fields are now determined in Rust, using a single regex
instead of the overkill stripHTMLMedia(). The old implementation
has been moved into the Pystache code for now.
