Feature: Add campaign pages

[kjohnson]

Resolves GIVE-1150

Description

This PR introduces the Campaign Page, which can be customized via the Block Editor.

From the campaign list table (coming soon), the campaign page can be accessed via /wp-admin/?action=edit_campaign_page&campaign_id={id}.

The Campaign Page model is created "on demand" when attempting to customize the page content (see the testing instructions). Ideally, this provides us flexibility to update the "default" state of the page when not customized (as blocks can be complicated to update if created ahead of time).

Note: This does not include front-end routing of the Campaign for publicly rendering the page content.

Affects

The Campaign/Page relationship has been moved to the CampaignPage class, which now stores the related campaignId as a property (as opposed to the Campaign having a pageId property). This prevents broken references to a CampaignPage when the underlying post is deleted or trashed.

Visuals

Testing Instructions

With a Campaign created (manually or via the database), go to /wp-admin/?action=edit_campaign_page&campaign_id={id} to be redirected to the related campaign page in the Block Editor.

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

[alaca]

Looks good to me 👍

[glaubersilva]

@kjohnson Great work! Just left a couple of comments related to small string tweaks.