Removed downlload delay setting for subscriptions

Subscription downloads already queued are now not requeued on the next check

Headers in download queue table are now sortable

Added button to clear all finished downloads in the downloads manager

backend/appdata/default.json

@@ -42,8 +42,7 @@
       "allow_subscriptions": true,
       "subscriptions_base_path": "subscriptions/",
       "subscriptions_check_interval": "300",
-      "redownload_fresh_uploads": false,
-      "download_delay": ""
+      "redownload_fresh_uploads": false
     },
     "Users": {
       "base_path": "users/",