Consolidate user avatar Vue logic

Creates 3 vue components:

1. user-avatar-link
2. user-avatar-image
3. user-avatar-svg

and implements them in several places.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31017

• Port to EE
• Test each implementation manually

EE Compatible Branch: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1699

spec/javascripts/vue_shared/components/user_avatar_link_spec.js

@@ -35,7 +35,7 @@ describe('User Avatar Link Component', function () {
   });
   it('should render <a> as a child element', function () {
-    expect(this.userAvatarLink.$el.querySelector('a')).not.toBeNull();
+    expect(this.userAvatarLink.$el.tagName).toBe('A');
   });
   it('should have <img> as a child element', function () {