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/attrs_directive.js

-import Vue from 'vue';
-import _ from 'underscore';
-
-function setAttrsFromBinding(el, binding) {
-  _.each(binding, (val, key) => {
-    el.setAttribute(key, val);
-  });
-}
-
-const AttrsDirective = Vue.directive('attrs', {
-  inserted(el, binding) {
-    setAttrsFromBinding(el, binding);
-  },
-  updated(el, binding) {
-    setAttrsFromBinding(el, binding);
-  },
-});
-
-export default AttrsDirective;