Preserve previous data on create datatype form error

[madprime]

Description

When creating a new DataType on the site, I produced a form error – and all my edits were lost (including extensive text). When rejected, a form should reload filled with the information people entered when submitting – losing this can be frustrating, and lead to errors.

I checked the "edit" form and it's fine – just the "create" form. This update adds the template to load submitted information into the form, if it exists.

Testing

• passed automated testing locally
• ran locally to test manually:
  • reproduced bug
  • checked all fields are now preserved on form error & reload
  • checked form submission works once errors are corrected