Unify server announcement formatting between CLI and WebUI

cc: @yomybaby @adrysnupstream: 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](https://rich.readthedocs.io/en/latest/), and it seems to strip all the remaining text once an HTML tag appears.

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

raw_doc = """
# header
plain text

html paragraph

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_.

JIRA Issue: BA-25