This also styles those inner .banner_message elements to lose
margin inherited from Bootstrap. (This is now also applied to
the upload-message banner.)
It's better to achieve that with a class selector; using a `p`
element selector would mean that such a style would be evaluated
for all `<p>` elements in the DOM. Which is of course a whole lot,
thanks to Markdown alone.
Fixes: #26922
We add a new banner informing the user if and when they send a message
to a muted topic / stream. It also has a button to unmute the topic.
Fixes: #24246.