Rollback image-webpack-loader update

[cgsunkel]

Description of change

After we updated image-webpack-loader our build started breaking everytime a PR was merged. This is due to a deep dependency of image-webpack-loader called gifsicle. For an unknown reason, the devs of gifsicle decided to remove the Linux binaries, which is causing our Jenkins build to break (see here and here for more information).

This PR rolls back image-webpack-loader to version 6.0.0, which will hopefully resolve our build problems.

Checklist

• Has the branch been rebased to master?
• Automated tests (Any of the following when applicable: Unit, Functional or Acceptance)
• Manual compatibility testing (Browsers: Chrome, Firefox, IE11, Safari)