Changed whitelisting to use channel JSON instead of page scraping.

It also only pulls data once. Uses channelID now instead of URLs.

Resolves https://github.com/ajayyy/SponsorBlock/issues/275

TODO: Add migration method to use channelID instead of channelURL

public/_locales/en/messages.json

@@ -540,5 +540,8 @@
     },
     "hiddenDueToDuration": {
         "message": "hidden: too short"
+    },
+    "channelDataNotFound": {
+        "message": "Channel ID not loaded yet"
     }
 }