Add illustrations for survey response landing page

Design in Figma

https://www.figma.com/file/U4HG8DlMsBactEiVA3Nkg4/Add-Ease-Score-and-Interview-Recruitment-Track-to-Onboarding-Email-Series?node-id=13%3A313

Purpose

This MR is needed for gitlab#341090 (closed). It adds illustrations that will be used for the updated survey response landing page:

Screenshots

chat.svg

gift.svg

Checklist

For a new icon and/or illustration:

• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.