Fix Ember not re-using DOM elements on block updates

Causing serious performance issues without the stable key

app/components/contribution-list/component.js

@@ -73,6 +73,7 @@ export default Component.extend({
           .filter(Boolean);
 
         return EmberObject.create({
+          key: raw.groupId || `c-${first.id}`,
           groupId: raw.groupId,
           items,
           contributors,

app/components/contribution-list/template.hbs

@@ -30,7 +30,7 @@
 {{/if}}
 
 <ul class="item-list contribution-list {{if @loading 'loading'}}">
-  {{#each this.groupedContributions as |group|}}
+  {{#each this.groupedContributions key="key" as |group|}}
     {{#if group.isGrouped}}
       <li role="button" data-group-id={{group.groupId}}
           {{action "toggleGroup" group}}
@@ -77,7 +77,7 @@
         {{/each}}
       {{/if}}
     {{else}}
-      {{#each group.items as |contribution|}}
+      {{#each group.items key="id" as |contribution|}}
         <li role="button" data-contribution-id={{contribution.id}}
             {{action "openContributionDetails" contribution}}
             class="{{item-status contribution}}{{if (eq contribution.id @selectedContributionId) " selected"}}">