Add more feature specs for work item assignees

Currently, we don't have enough feature spec coverage for work item assignees. In order to cover the most ordinary cases, we have to ass these tests:

• assigns the current user by "Assign myself" button
• assigns multiple users when multiselect is enabled (EE)
• reassigns to another user from the current assignee when multiselect is disabled (FOSS)
• removes all users on clicking the cross icon
• removes a certain user from the list on clicking the assignee cross icon
• disables assignees widget for unauthorized users
• updates assignees in real-time