Add annotations support for DWP YouTube channel

In order to link directly from YouTube videos to GOV.UK the account
holder needs to associate a site with their content. This is done via
the standard Google site verification method.

More info here:

https://support.google.com/youtube/answer/2887282?hl=en-GB
Merge pull request #426 from alphagov/add-dwp-annotations-verification

Add annotations support for DWP YouTube channel