Add service load_config_discard_changes

[FSund]

Description

Adds a service load_config_discard_changes that avoids the "There are unsaved changes" dialogue that appears when calling the regular load_config service.

I am very open to alternative names for the service, and alternative implementations.

Implements suggestions in #1709

Checklist

• If you are addressing rendering issues, please provide:
  • Images of both, broken and fixed renderings.
  • Source code to reproduce the issue, e.g. a YAML or rosbag file with a MarkerArray msg.
• If you are changing GUI, please include screenshots showing how things looked before and after.
• Choose the proper target branch: latest release branch, for non-ABI-breaking changes, future release branch otherwise.
  Due to the lack of active maintainers, we cannot provide support for older release branches anymore.
• Did you change how RViz works? Added new functionality? Do not forget to update the tutorials and/or documentation on the ROS wiki
• While waiting for someone to review your request, please consider reviewing another open pull request to support the maintainers of RViz. Refer to the RViz Wiki for reviewing guidelines.

[rhaschke]

Generally looks good. Given that this targets an already released distro, I agree with your choice of using another service name. Otherwise, the existing message should have been augmented by an extra flag.
Please address the two minor comments below.

[FSund]

Thanks for the speedy reply and review. Your comments have been addressed in the latest commits.

Do I need to do anything about the failing CI jobs?

[rhaschke]

Some more nitpicks.

[rhaschke]

Thanks a lot. I'm happy if CI is...