Problem:

Users are easily scared by the big red banner  "Jenkins is going to shut down".

Once a new user sees this banner, they immediately write us admins to know whats happening. Once we tell them that their jobs are not cancelled and can continue, they are relaxed.

Users should know that their jobs are (usually) not aborted and will continue. 

This is especially true for "Safe Restarts", where the controller is back very fast. We want to let users know why we do a safeRestart and that the jobs can continue. 

Furthermore, it should be possible to define the full restart message and not just the reason. I might go into "prepare shutdown" mode without shutting down but preparing a restart or similar.

Other improvements:

• Added "Cancel" button for safeRestart mode.
• Added hint on the restart page for safe restarts.
• Improved messages to be more clear on what is happening.
• Less aggressive banner for safe restarts.