Permit exit page heading and markdown for condition endpoint

[SamJamCul]

What problem does this pull request solve?

https://trello.com/c/MdPcqbFI/2154-allow-form-creators-to-add-an-exit-page-to-a-route

Allow exit_page_heading and exit_page_markdown when interacting with the conditions endpoint. It's necessary for the exit page work!

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]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud