discard stylesheets etc in HTML snippets

[snarfed]

My user page shows this right now:

That post, https://snarfed.org/2023-06-02_bridgy-stats-update-8 , has HTML content in its stored Object, which is fine, except it starts with an inline <style>, and the snippet is including its text. We should sniff for HTML in snippet generation and drop <style> tags, and maybe others.

https://github.com/snarfed/bridgy-fed/blob/d9cd5d14b9a8156b48574c9246a44ea053fa04ea/pages.py#L216-L218

The only general purpose sniffing code I see in granary right now is in _content_for_create: https://github.com/snarfed/granary/blob/7724cb29457b74a7ae961eaaedf6cf0326fc4362/granary/source.py#L805-L815