FEATURE: Improve backup stats on admin dashboard

* Dashboard doesn't timeout anymore when Amazon S3 is used for backups
* Storage stats are now a proper report with the same caching rules
* Changing the backup_location, s3_backup_bucket or creating and deleting backups removes the report from the cache
* It shows the number of backups and the backup location
* It shows the used space for the correct backup location instead of always showing used space on local storage
* It shows the date of the last backup as relative date

app/assets/javascripts/admin/components/admin-report-storage-stats.js.es6

@@ -0,0 +1,40 @@
+import { setting } from "discourse/lib/computed";
+import computed from "ember-addons/ember-computed-decorators";
+
+export default Ember.Component.extend({
+  classNames: ["admin-report-storage-stats"],
+
+  backupLocation: setting("backup_location"),
+  backupStats: Ember.computed.alias("model.data.backups"),
+  uploadStats: Ember.computed.alias("model.data.uploads"),
+
+  @computed("backupStats")
+  showBackupStats(stats) {
+    return stats && this.currentUser.admin;
+  },
+
+  @computed("backupLocation")
+  backupLocationName(backupLocation) {
+    return I18n.t(`admin.backups.location.${backupLocation}`);
+  },
+
+  @computed("backupStats.used_bytes")
+  usedBackupSpace(bytes) {
+    return I18n.toHumanSize(bytes);
+  },
+
+  @computed("backupStats.free_bytes")
+  freeBackupSpace(bytes) {
+    return I18n.toHumanSize(bytes);
+  },
+
+  @computed("uploadStats.used_bytes")
+  usedUploadSpace(bytes) {
+    return I18n.toHumanSize(bytes);
+  },
+
+  @computed("uploadStats.free_bytes")
+  freeUploadSpace(bytes) {
+    return I18n.toHumanSize(bytes);
+  }
+});