case/correspondence: Add automatic task on new prisoner correspondence

This allows for designating a list of users (via a configuration key) per prison, so that when new incoming prisoner mail is received, a user is randomly chosen from the list assigned to that prison, and a task is assigned to them.

Configuration key "correspondence-task-prison-users" is a JSON hash of String -> Array<UserId> where the key is the prison ID as a string, and the value is an array of user IDs to choose from when assigning a task. If a key is not present for a given prison, or the array for that prison is empty, a task is not created.