Alert Management assignee avatar

What does this MR do?

A tiny MVC for #223846 (closed) where we:

• Add the assignee avatar to the Alert Management list view
• Add the avatar to the Alert Details sidebar
• Do a small bit of clean up on some bad code for the sidebar assignee component.

This merge will be followed with a D.R.Y merge between ~"Category:Alert Management" and Category:Incident Management to help reduce overall code duplication for shared items like this.

Screenshots

List View	Details View

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #223846 (closed)