Feature: add new link for the "Add Form" admin menu to create forms with the new visual form builder

[glaubersilva]

Resolves GIVE-998

Description

This PR replaces the default "Add Form" link added by the WP Core for custom post types with a new link that uses the visual form builder as default editor.

As we don't have any filter to change this value, the global $submenu; var was used to accomplish that.

Affects

The "Add Form" admin menu link.

Visuals

Here is a sample of how the global $submenu; var looks like:

Testing Instructions

1. Open the WP Admin area;
2. Click on the "Add Form" submenu from Give;
3. A new form using the Visual Form Builder should be created.

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

[glaubersilva]

@pauloiankoski This is ready for re-review.

[rickalday]

Pass manual QA tests.