Resolve "New Purchase Congratulation Illustration"

Purpose

The current purchase illustration is old, outdated and needs a lot of love. This illustration is shown to customers immediately after a successful purchase. Some more context can be found in @jelder's comment.

Screenshots

Proposal

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• 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.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.

Closes #399 (closed)