Add a separate route for submitting file upload answers

[stephencdaly]

Warning

Do not merge until the WAF rules have been updated to allow large request bodies for the new file upload route (as well as the save question route)

What problem does this pull request solve?

We need to be able to exempy file upload requests from particular WAF rules:

• the file size rule so requests can include files >8KB in size
• the XSS rule, so that files that contain XML, such as PDFs are not blocked.

So that we don't need to allow these rules for submitting all answer types, make it so that file upload requests are submitted to a dedicated route.

For now, this still allows file uploads to be submitted to the standard save question route so we don't cause issues for users currently filling out forms, but this will be changed in a separate deployment.

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?
• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

[github-actions]

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