Add Campfire logo

Purpose

We are adding Campfire integration logos, as per Add integration logo: Campfire (gitlab#367607 - closed)

The icon has been importer from http://37assets.s3.amazonaws.com/svn/37signals-logos.zip, and per brand guidance.

Screenshots

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.