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.

tests/integration/components/user-avatar/component-test.js

@@ -0,0 +1,14 @@
+import { module, test } from 'qunit';
+import { setupRenderingTest } from 'ember-qunit';
+import { render } from '@ember/test-helpers';
+import hbs from 'htmlbars-inline-precompile';
+
+module('Integration | Component | user-avatar', function(hooks) {
+  setupRenderingTest(hooks);
+
+  test('it renders', async function(assert) {
+    await render(hbs`{{user-avatar}}`);
+
+    assert.equal(this.element.textContent.trim(), '');
+  });
+});