Allow multiple assigned users per task

# Problem

We'll need to change the `task-assignment` component to:

- allow selecting multiple users
- render all the selected users in the dropdown
- render all the selected users as a list of image icons in the selected component

Start by locating and reading the task-assignment component, then trace how it stores and renders the selected user. The work is complete when multiple users can be selected, all selected users appear in the dropdown, and the selected component shows their image icons as a list.

Written by the indexing model from the issue text.