Consolidate user avatar Vue logic

We construct user avatars and user avatar links in different ways across Vue components. We need to DRY that up. Needed as part of https://gitlab.com/gitlab-org/gitlab-ce/issues/30882

MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10718

added frontend label

mentioned in issue #24339 (closed)

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/30286

mentioned in merge request !10718 (merged)

Adding a few more specific places we should add this refactored logic

• app/assets/javascripts/sidebar/components/assignees/assignees.js (!1456 (merged))

mentioned in merge request !11302 (closed)

closed via merge request !10718 (merged)