As a user, I want to be able to choose whether or not the Test Runs get a default Assignee

As a user, I should be able to choose whether or not a default Assignee will be used for newly created Test Runs. If the default Assignee option is disabled, then the Test Runs should be unassigned by default.

The way it stands at the moment, when a Test Execution is created, all Test Runs associated with it get assigned to a single person, who might not be the same person that will execute the tests, leading to a lot of unnecessary notification emails and the need to overwrite the Assignee later. Having the option to keep the Test Run unassigned by default could lead to a cleaner and more natural workflow in some instances.