refactor: accommodate header image for 2-panel layout

[JoshuaHungDinh]

Resolves GIVE-150

Description

This PR includes the necessary changes to accommodate the header image while using the 2-panel layout.

Additional changes include:

• Remove margin from container that was clipping the border-radius on the form.
• Remove media max-width on the header panel as it was applying in the canvas but not on the frontend resulting in inconsistent views.
• Replace border-radius in design settings stylesheet with --givewp-rounded-8

Affects

HeaderImage design setting

Visuals

https://github.com/impress-org/givewp/assets/75056371/51874de2-7377-46ce-9ed7-253db0e6c016

Testing Instructions

• Create a form in the visual form builder
• Set layout to two-panel
• Upload a header image
• Test all 3 image styles
• Toggle the various header settings to ensure the layout does not break
• Verify previous layouts do not break

Pre-review Checklist

• [x] Acceptance criteria satisfied and marked in related issue
• [x] Relevant @unreleased tags included in DocBlocks
• [ ] Includes unit tests
• [x] Reviewed by the designer (if follows a design)
• [x] Self Review of code and UX completed