Changed the blacklist to an allowlist - Resolves #196

[Veviser]

Changed the blacklist to an allowlist, letting users opt-in to sites explicitly. Added Youtube to the initial offering. Changed settings page to indicate this. Did not version bump.

Note that settings appear out of sync and show everything previously in the blacklist to be in the allowlist. This can be addressed by saving the settings with the appropriate details or deleting the temporary files associated with the extension prior to reinstalling. New downloads will not be affected.