Migrate spec/javascripts/sidebar/ to Jest

Description

Let's migrate the specs in spec/javascripts/sidebar/ from Karma to Jest.

See the related epic for how to do this.

• spec/javascripts/sidebar/mock_data.js !32535 (merged)
• spec/javascripts/sidebar/subscriptions_spec.js !30500 (merged)
• spec/javascripts/sidebar/sidebar_assignees_spec.js !29461 (merged)
• spec/javascripts/sidebar/lock/lock_issue_sidebar_spec.js !32535 (merged)
• spec/javascripts/sidebar/lock/edit_form_buttons_spec.js !30500 (merged)
• spec/javascripts/sidebar/assignee_title_spec.js !24919 (merged)
• spec/javascripts/sidebar/sidebar_move_issue_spec.js !32535 (merged)
• spec/javascripts/sidebar/components/time_tracking/time_tracker_spec.js !32535 (merged)
• spec/javascripts/sidebar/sidebar_subscriptions_spec.js !30500 (merged)
• spec/javascripts/sidebar/participants_spec.js !30500 (merged)
• spec/javascripts/sidebar/sidebar_mediator_spec.js !30500 (merged)