Allow setting process assignee as candidate assignee for task(s)

Several users would like to configure tasks in the proces template to automatically be assigned to the user which is responsible for the whole process.

As a user, I currently can set in the "task" section of a process template a user which should be automatically set as responsible as soon as the task becomes active.

In future, a user should be able to mark, that the user who is currently responsible for the workflow will automatically be set as responsible for the task as soon as the task becomes active.