Improve styles for group messaging.

• Styling of the conversations and the category titles. E.g. subtitles (group messaging) and the unread badge look quite bad.
• Check if you can make the view to fill 100% of height max (like web does) so the category titles are always visible.
• We should display the online status (I think you'll do that in another issue). There's a comment in the view for this: @todo: Display connection status
• Don't style the contacts button yet, let's see what Barbara decides in the end.