Add support for third-party PNG illustrations

Purpose

Add support for third-party PNG illustrations

Due to compliance requirements some third-party illustrations might be PNGs. This adds support for them if they are in the illustrations/third-party-logos sub directory.

Note: This MR purposefully does not include a new PNG icon and only the code needed for icons. In order to properly review this MR, please check !810 (merged) which also adds a new icon.

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.