Add user activity system

ember/app/components/user/activity-item.js

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+import Ember from 'ember';
+
+import FadeIn from 'flarum/mixins/fade-in';
+
+export default Ember.Component.extend(FadeIn, {
+  layoutName: 'components/user/activity-item',
+  tagName: 'li',
+
+  componentName: Ember.computed('activity.type', function() {
+    return 'user/activity-'+this.get('activity.type');
+  })
+});

ember/app/components/user/activity-post.js

@@ -0,0 +1,9 @@
+import Ember from 'ember';
+
+export default Ember.Component.extend({
+  layoutName: 'components/user/activity-post',
+
+  isFirstPost: Ember.computed('activity.post.number', function() {
+    return this.get('activity.post.number') === 1;
+  })
+});