Previously livestreams were driven by a `livestream` tag and separate
`livestream enabled`
and `livestream_chat_allowed_groups` site settings. Now that we're
integrating livestream into events we don't really need these anymore.
This change makes livestream a toggle on any event with a supplied URL.
If chat is disabled we explain why livestream can't be enabled.
Livestream becomes an attribute of an event rather than relying on a
tag.
When a URL is included and livestream is toggled on, the link oneboxes
with the event
<img width="800" alt="image"
src="https://github.com/user-attachments/assets/893671f1-202c-4023-80a8-f9b6f8285988"
/>
<img width="500" alt="image"
src="https://github.com/user-attachments/assets/3a56f038-f4c7-4c68-a0cd-026c47327ce5"
/>
<img width="2434" height="1606" alt="image"
src="https://github.com/user-attachments/assets/436c54fa-529e-4b8c-903a-5ecf3b6afe84"
/>
---------
Co-authored-by: Martin Brennan <martin@discourse.org>
