Add tag for new subsection "Dealing with HMRC"

This adds a migration which creates a new section tag for "Dealing with 
HMRC", a child of the "Money and tax" section.
Tagging content with dealing-with-hmrc

Relates to https://www.pivotaltracker.com/story/show/67290772. Tags
provided content with the new tax/dealing-with-hmrc section tag.
Merge pull request #140 from alphagov/dealing-with-hmrc-subsection

Dealing with hmrc subsection

https://www.pivotaltracker.com/story/show/67290772