Add an option to hide some buttons in case the policy is discontinued

[Neurone]

Problem description

When a policy is discontinued, the backend (correctly) doesn't allow any other operations.
On the frontend this seems to be an error because I can still see the buttons for the operations and when I use them I get a 500 error (see screenshots).

Requirements

• Policy developers can flag a button as available only if the policy is not discontinued.
• Buttons flagged as hideWhenDiscontinued won't be displayed in the webUI when the policy is discontinued.

Definition of done

• Buttons in the buttonBlock contain a special property to set visibility in case of discontinued policy.
• WebUI respects the options set at individual button level