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_image.js

+import defaultAvatarUrl from 'images/no_avatar.png';
 import UserAvatarSizeMixin from './user_avatar_size_mixin';
 export default {
@@ -7,7 +8,7 @@ export default {
     src: {
       type: String,
       required: false,
-      default: gon.default_avatar_url,
+      default: defaultAvatarUrl,
     },
     cssClasses: {
       type: String,