Allow destroying of blank html versions
process reasons as they appreas as markdown

https://govuk.zendesk.com/agent/#/tickets/156747
Fix importer by adding back in the reject_if condition
Skip if the record is marked_for_destruction
Controller test to ensure the param is set

And the html version is destroyed

NS: you always need to pass an ID even with a has_one when using nested
attributes
Make unpublish explanation previewable
Use edition methods for setting first_published_at

And fix the logic for first_public_at

Data migration to clean up any publications without a first_published_at

publication_date is always set and is already a datetime
Merge pull request #621 from alphagov/unpushished-reason-in-markdown

https://www.pivotaltracker.com/s/projects/367813/stories/52387669

Unpublish reasons support markdown
Typo

https://www.pivotaltracker.com/story/show/51904309
Fix indentation
typo

https://www.pivotaltracker.com/story/show/52622505
Merge pull request #625 from alphagov/destroy-html-version-of-publication

https://www.pivotaltracker.com/story/show/51986337

Destroy html version of publication
Merge pull request #630 from alphagov/bug-fixes

Typos

https://www.pivotaltracker.com/story/show/51904309
https://www.pivotaltracker.com/story/show/52622505
Merge pull request #604 from alphagov/fix-first_published_at-for-publications

Related to https://www.pivotaltracker.com/story/show/51549341