Update the manifest section of the submission form to handle links to the remote files

We primarily create metadata in the form, but for usability it would of course be nice to have a more direct link to the source.

Instead of just giving a name and description, there would be an additional field "preview URL" that users can fill out manually. (When this is done, create an issue with all the ideas for some "magic" here, like just pointing to a repo and then scanning for a codecheck.yml and generating the information of the preview URL on the fly).

The result can look like an intermediate development version (that featured a file upload we do not need):

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User story:

_As an editor, I would like to have a direct link to the manifest files from the OJS backend so that I can easily compare them with the originals from the submission without familiarising myself with the specific data/code repository where they are saved._
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## Related User Stories

- A-ME1 (related): As an author, I want to enter CODECHECK-related metadata (code URL, data URL, expected outputs) only once...
- A-SO3 (related): As an author, I want to indicate during submission whether my manuscript includes computational workflows...
- A-GV1 (related): As an author, I want inline guidance, examples, and validation in the submission form...
- R-SC3 (related): As a reader, I want the article's landing page to provide structured, direct links to the verified code repository...
- A-ME2 (related): As an author, I want the submission form to include a checklist of reproducibility requirements...

_User story references from the [CHECK-PUB User Stories document](https://codecheck.org.uk/pub/)._