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: {
+    avatarSizeStylesMap() {
+      return {
+        width: `${this.size}px`,
+        height: `${this.size}px`,
+      };
+    },
+    avatarSizeClass() {
+      return `s${this.size}`;
+    },
+  },
+};