Upgrade sanitize dependency to 2.1.0

The older version used by govspeak (2.0.3) contains a bug where colons in ids would not be allowed, which prevents footnotes being saved as part of a document body in specialist-publisher.

This was fixed in 2.1.0 (see https://github.com/rgrove/sanitize/pull/87).

Trying to force an upgraded version of sanitize to be activated using the specialist-publisher Gemfile is a non-starter as bundle refuses to upgrade to that version as it's too far ahead of Govspeak's requirements.

Now that this change has been merged in I'm also going to open a whitehall PR to move off an alphagov fork of sanitize back to the official gem.