Feature: add campaigns menu item and initial empty page

[glaubersilva]

Resolves GIVE-1122

Description

This PR adds a submenu item under the Give main menu and an initial empty page where the new campaigns list table will be loaded.

Affects

The Give admin menus/submenus.

Visuals

Testing Instructions

1. In the admin area click on the new Give > Campaigns item;
2. Make sure a new empty page with the "The campaigns list table will be loaded here..." temp text is loaded.

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