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

@@ -30,7 +30,7 @@ describe('User Avatar Size Mixin', () => {
     });
   });
-  fdescribe('#avatarSizeStylesMapString', () => {
+  describe('#avatarSizeStylesMapString', () => {
     it('should be a defined computed value', () => {
       expect(this.vueInstance.avatarSizeStylesMapString).toBeDefined();
     });