Add fieldname and username to assignee avatar tooltips in work items

Summary

In the issue list and in legacy child items, hovering an assignee avatar shows a tooltip including "Assignee", the display name, and the username.

Noting this is "Assignee" is helpful as it's the only field label in this context.

Overflow badge tooltip in these cases is <#> more assignees

These details should be added to:

• Work item child items
• Work item development widget MRs

Before	After