As a user, I can assign test runs to users

It should be possible to assign users to test runs and not just only test execution issues.

The test runs table in the test execution view issue screen must contain an additional column for the assignee and the must be an action for assigning a test run to a single user.

The default assignee for test runs is the assignee of the test execution issue.

The assignee for a test run can be empty if the Test execution issue is unassigned.