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/boards/issue_card_spec.js

@@ -129,7 +129,7 @@ describe('Issue card component', () => {
       it('sets title', () => {
         expect(
-          component.$el.querySelector('.card-assignee a').getAttribute('title'),
+          component.$el.querySelector('.card-assignee img').getAttribute('title'),
         ).toContain(`Assigned to ${user.name}`);
       });