The status is shown for
- The author of a commit when viewing a commit
- Notes on a commit (regular/diff)
- The user that triggered a pipeline when viewing a pipeline
- The author of a merge request when viewing a merge request
- The author of notes on a merge request (regular/diff)
- The author of an issue when viewing an issue
- The author of notes on an issue
- The author of a snippet when viewing a snippet
- The author of notes on a snippet
- A user's profile page
- The list of members of a group/user

Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/39608

Use SVG icons to avoid having to position things
See https://gitlab.com/gitlab-org/gitlab-ce/issues/39608#note_50088917

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32801

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32551

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32449

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32087

Easy reproduction:

 1. Edit an issue title
 1. Notice the system note added
 1. Switch to another tab and back (to fire the polling immediately)
 1. Notice the flicker (without this fix)