Add an extension for when a user requests a deployment

There is a decision of whether to put this before or after the permissions check.
I opted for after, since of the potential security implications.
A safe default saves peoples lives.
When we introduce pipelines, this won't be the only point at which deployments can be initiated. Check https://github.com/tractorcow/deploynaut/blob/experiments/pipelines/code/control/DNRoot.php#L527