client-apps: Improvements to web client shortcuts

Make the following changes to reduce clutter and allow easy access to web clients from the configuration pages of apps.

• When a self-hosted webapp is available, show Launch button beside the Client Apps button in addition to showing it inside the table in Client Apps
• When Client Apps table only contains a single self-hosted web app, show only the Launch button in place of the Client Apps (to avoid redundancy)
• Change background color of all web apps inside the Client Apps table to white background, since the self-hosted web apps are already highlighted outside.
• If a button to launch the web client is available, remove the redundant hyperlink from the description (looks like /app)

Hint: Self-hosted web apps (running on the FreedomBox device, like tt-rss) have relative urls while external web apps like Riot have full urls in the data from manifest.py