refactor: collapse encrypt preview submission middleware

[mantariksh]

Combines the middleware for the endpoint which handles storage mode preview submissions.

Closes #1435

Other improvements

While writing integration tests, I noticed a couple of issues with the Joi validation shared by the actual submission and preview submission endpoints:

1. helper.error results in Joi returning an error message, because they require a custom string (e.g. 'any.custom') to be passed as the first argument, rather than our own error message.
2. The encryptedFile key in individual attachments was not a required key, even though it should be.

These two issues have been fixed in encrypt-submission.middleware.ts.

Tests

• Create a storage mode form and add an email field with email confirmation. Submit it in preview mode. You should not be able to see the response in the data tab, but you should receive the email confirmation.
• Change the authType to SP and submit in preview mode.
• Change the authType to CP and submit in preview mode.

[karrui]

lgtm 🎉