Warning user of network issue while filling/submitting form

## Description

When a person filling out a form loses their Internet connection and ends up going offline in the middle of filling or submitting their form, we could prompt them to save their answers (if this feature is enabled).

This issue is broader than the file upload feature (raised in issue #5990), and thus requires a separate issue to address it with a more holistic solution.

## Options to consider (both when filling and submitting, including data and files)

- Add a banner or indicator to let users know they are offline.
- Saving progress does not apply to all forms, and requires the server, so we might need to consider an offline state for this action to preserve user-filled data.
- HMW consider going offline anytime in the process. An offline option to save and return later?

## Previously