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

app/assets/javascripts/vue_shared/components/user_avatar/user_avatar_size_mixin.js

 export default {
   computed: {
     avatarSizeStylesMapString() {
-      return this.size ? `{ width: '${this.size}px', height: '${this.size}px';` : '{}';
+      return this.size ? `{ width: '${this.size}px', height: '${this.size}px' }` : '{}';
     },
     avatarSizeClass() {
       return this.size ? `s${this.size}` : '';