Make feeds validate and add change note to entries

- Feeds were not valdating due to us incorectly sending categories with
  entries. The correct way is to use an attribute on a category element
  not put the value in the element.
- Add a change note if there is one to the entry body. For this it was
  needed to send the govdelivery version as html rather than text as the
  atom spec lets parsers remove whitespace in the content tag.
- Having the content always go as HTML enabled reducing the test logic
  in each of the tests and moving more into the helper method. This
  makes the tests slightly cleaner to look at.
Merge pull request #160 from alphagov/add-change-note-to-atom

Make feeds validate and add change note to entries