feat: encryption boundary shift in preview mode

[kathleenkhy]

Problem

Adding encryption boundary shift for storage mode form submissions previews.

Closes FRM-1506

Solution

added encryption shift to preview mode - also reducing the complexity of implementing encryption within preview mode by no longer saving any answers or attachments uploaded in preview mode (formData will be saved as an empty object {}).

This decision was made because:

• use of preview mode, let alone uploading attachment functionality or sending of answers to users' emails (via email confirmation if selected as a field) within preview mode is low
• main value/ usage of preview mode is primarily used for testing what the look and feel of the form would look like, rather than viewing the answers/ attachments saved in the preview mode after submitting a form

Breaking Changes

• [ X] No - this PR is backwards compatible

Manual Tests

1. To check that no data (i.e. an empty object {}) is sent to the Backend

• Enter Preview mode in Admin view (by clicking the eye button on the top right hand corner)
• Enter your Network tab
• Fill in form fields in the preview mode
• Upon clicking submit, check in the Network tab that the storage call has an empty object {} under Payload

2. To check that the preview banner warns the user that no answers or attachments uploaded will be saved

• For non-payment forms, the banner should show:

You will not be able to view submitted answers or attachments in Form Preview mode. Open your form to test a form submission.

• For payment forms, the banner should show:

You will not be able to make a test payment, or view submitted answers or attachments in Form Preview mode. Open your form to make a test payment or form submission.

[linear]

FRM-1506 encryption boundary shift for storage mode form submissions previews

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #6969. Mergify cannot evaluate rules on this PR. ⚠️

[tshuli]

for the deleted tests, email previews should not be affected - could you please reinstate them and check if they are failing?

otherwise lgtm!

[tshuli]

thanks for making the edits @kathleenkhy, lgtm! and happy cny too 🧧🧧🧧

please write some manual tests for the PR description before merging in so we can test during release 😊