Add empty what happens next markdown field

[DavidBiddle]

What problem does this pull request solve?

Trello card: https://trello.com/c/AbPj2LRr/1191-allow-markdown-on-confirmation-page-what-happens-next-content

Adds a null what_happens_next_markdown field to the form. Also adds this null field to the most recent made_live_form.

To do in other PRs:

• add logic in the runner (Render what happens next markdown if it's present forms-runner#496) and admin to use the markdown field if it's present, and the HTML if not
• convert the HTML content in the API into markdown and store it in this new field
• cleanup: delete the old field and all the logic related to it which we no longer need

Things to consider when reviewing

• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Has all relevant documentation been updated?

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
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No Coverage information
0.0% Duplication

[aliuk2012]

Does this have to change? https://github.com/alphagov/forms-api/blob/main/db/seeds.rb#L85

[DavidBiddle]

@aliuk2012 I don't think so at the moment - the seed still works and sets what_happens_next_markdown to null in the database, which is the default value (which we want because that's the state the forms in prod will be in after the migration). Good spot though - I'll add a note to the card so that I remember to update the seed when we do the conversion to markdown.

[aliuk2012]

I've tested this locally and everything looks good. I tried using form-runner from main branch and also the feature branch and both cases worked perfectly.