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

test/javascripts/fixtures/dashboard-next-general.js.es6

@@ -1,13 +1,6 @@
 export default {
   "/admin/dashboard/general.json": {
     reports: [],
-    last_backup_taken_at: "2018-04-13T12:51:19.926Z",
-    updated_at: "2018-04-25T08:06:11.292Z",
-    disk_space: {
-      uploads_used: "74.5 KB",
-      uploads_free: "117 GB",
-      backups_used: "4.24 GB",
-      backups_free: "117 GB"
-    }
+    updated_at: "2018-04-25T08:06:11.292Z"
   }
 };