testbot/web: Allow customizing the TestBot's status.
This makes it possible to add news, tips, etc. Note that the web server typically does not have write access to the var/ directories (except for staging) which is why updating motd goes through the Engine.
The news and tips would only amount to a handful of one-liners to not clutter the top of the page too much. But they would allow warning of VM configuration changes or advertising new functionality in a place where TestBot users are most likely to see the information.