Show a dot for unreads if channel is muted

Just as in the web app we need to show a dot indicator on the subscription item if it is muted and has unreads.
<img width="282" alt="image" src="https://github.com/zulip/zulip-flutter/assets/63330019/ca9f5162-3ba6-485e-ae4b-215caf7c1961">
## Related issues
- #346
