Added option to rate limit downloads

Added option to force delay between videos in a subscription

Fixed issue where file handle was maintained on files deleted through unsubscribing

backend/appdata/default.json

@@ -12,7 +12,8 @@
       "custom_args": "",
       "safe_download_override": false,
       "include_thumbnail": true,
-      "include_metadata": true
+      "include_metadata": true,
+      "download_rate_limit": ""
     },
     "Extra": {
       "title_top": "YoutubeDL-Material",
@@ -40,7 +41,8 @@
       "allow_subscriptions": true,
       "subscriptions_base_path": "subscriptions/",
       "subscriptions_check_interval": "300",
-      "redownload_fresh_uploads": false
+      "redownload_fresh_uploads": false,
+      "download_delay": ""
     },
     "Users": {
       "base_path": "users/",