Integrate atc-mitigation with atc-react

[yugoslavskiy]

There are placeholders for Mitigation Systems (MS) in RA's requirements field. For now, it's not clear what is it. There is no link or any explanation. So we need to:

1. Develop Mitigation Systems entities in atc-mitigation
2. integrate them into atc-react, so the requirements will have links to the MS description
3. Develop a description of the relations