Add a (visual) hint when a build job is disabled

[steffen-wilke]

In Jenkins, it's possible to disable a build job which will block any execution or triggers, but still keep the job in place.
In WhatsON these jobs are currently displayed just like normal jobs and the user doesn't know that these jobs are not intended to run again.

We need to implement some visualization or hint that tells the user that a build job is disabled.

Suggestions:

• Gray icon with a hint, similar to when the job is offline/not existing anymore.
• Semi-transparent visualization of the while tree element.