Feature: Implement dynamic receipt settings

[jonwaldstein]

Description

This updates the confirmation receipt page to dynamically display the heading and description using the form builder's receipt settings.

Affects

The donation confirmation receipt view.

Visuals

Testing Instructions

Update the heading/description settings and make a donation to see the values displayed in the receipt.

Pre-review Checklist

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

[jonwaldstein]

@kjohnson this is ready for review.

Note: I think we should move the "Receipt and Thank You" settings into the Form tab. Reason being, they have no live preview at the moment - like the rest of the design settings - so it feels confusing in practice to be in the Design tab.

[kjohnson]

Looks good to me.