Implement "graceful shutdown" about toolkit HOT 6 CLOSED

We should send maintenance messages before shutting down the Toolkit; I think this is already implemented.

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No, it doesn't work because after reloads the message would be gone.
@anjestephens can you make the alert being persistent?

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If the Toolkit is about to shut down, every new page should just have this message displayed. We can do that in the backend without having to use websockets.

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With the new frontend we can easily implement this. There should also be a browser notification on maintenance alert.

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I implemented most of the backend functionality for this. @vikramalva we have to talk some more about where to store the maintenance state. Right now, I store it in the controller directly. This way, after a restart, the maintenance mode warning would go away and it also doesn't need the database. Otherwise, one would have to manually remove the maintenance mode after finishing maintenance.

ToDo (@joeldomke):

• Implement admin view to trigger the POST routes /api/backend/startmaintenance and /api/backend/endmaintenance (use BackendService for this)
• Disable Submit buttons when in maintenance mode

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We will postpone the admin view. Maintenance mode will be managed by calling the routes.
Do we want a browser notification?

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