Turn user avatars into a component

This allows us to add them anywhere in the app, and also to add more
features, like e.g. hover info boxes, links, and so on.

app/components/user-avatar/component.js

@@ -0,0 +1,10 @@
+import Component from '@ember/component';
+import { alias } from '@ember/object/computed';
+
+export default Component.extend({
+  contributor: null,
+  tagName: 'img',
+  classNames: ['avatar'],
+  attributeBindings: ['src'],
+  src: alias('contributor.avatarURL')
+});