Unify server announcement formatting between CLI and WebUI

[achimnol]

cc: @yomybaby @adrysn
upstream: Textualize/rich#3565

I found that one of our customer sites has a server announcement like:

The CLI interprets the server announcement as the Markdown formatter via Rich, and it seems to strip all the remaining text once an HTML tag appears.

import sys
from rich.console import Console
from rich.markdown import Markdown


raw_doc = """
# header
plain text
<p>html paragraph</p>
another line
"""
print("------------", file=sys.stderr)
print(raw_doc)
print("------------", file=sys.stderr)
console = Console(stderr=True)
doc = Markdown(raw_doc)
console.print(doc)

We need to ensure that either rich.markdown recognizes the HTML tags or the Web UI uses a proper Markdown renderer and let the customer admins write the server announcements in Markdown without mixing HTML.