Refactor tag building to be more easily extendable
Include local gov flag as a variation

Things that are relevant to local government are just one piece of the
matrix - they should be included in the combinations as everything else.
Include non-format specific variants of feeds

If a user just selects a topic, they won't get emails specifically about that topic
unless we notify these channels.
Merge pull request #483 from alphagov/allow-for-additional-govuk-delivery-tags

Allow for additional govuk delivery tags

https://www.pivotaltracker.com/story/show/47827853