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_size_mixin_spec.js

@@ -35,14 +35,6 @@ describe('User Avatar Size Mixin', () => {
       expect(this.vueInstance.avatarSizeStylesMapString).toBeDefined();
     });
-    it('should correctly transform size into width style', () => {
-      expect(this.vueInstance.avatarSizeStylesMapString).toContain(`width: '99px'`); // eslint-disable-line quotes
-    });
-
-    it('should correctly transform size into height style', () => {
-      expect(this.vueInstance.avatarSizeStylesMapString).toContain(`height: '99px'`); // eslint-disable-line quotes
-    });
-
     it('should return an plain object when size is undefined', (done) => {
       this.vueInstance.size = null;