FE: add user assignment dropdown to alert drawer

Why are we doing this work

• when a user is viewing alerts from threat monitoring alert's tab, the table only gives them a limited amount of information
• a user will be able to click on an alert row in the table and a drawer will appear, which will give them more information

Design

• https://www.youtube.com/watch?v=HuZgaMKdJF0&feature=youtu.be

Non-functional requirements

• Documentation: I am waiting until the end to update the documentation as this feature is behind a feature flag
• Testing

Implementation Plan

• first try to use the new assignee widget app/assets/javascripts/sidebar/components/assignees/sidebar_assignees_widget.vue
• otherwise make app/assets/javascripts/vue_shared/alert_details/components/sidebar/sidebar_assignees.vue resusable and reuse it