Integrate oekg-builder into the OEP repository

[adelmemariani]

Similar to the OEO-viewer, this tool is now integrated into the OEP repository (as a frontend of a Django app called 'factsheet'). Therefore, the code should be maintained in the OEP repository and this repository is going to be deleted in the future. In OEP, we only need to run the 'npm run build' command inside the 'front-end' folder of the 'factsheet' app. The compiled version of the front-end code is served in a Django template.

• [ ] Create a checklist with all steps to merge developments in this repo (AM)
• [ ] transfer issues
• [ ] ...
• [ ] delete old repo (LH)

[adelmemariani]

Commands like 'npm run build' should be moved to GitHub actions. I should invest some time in making a proper build process and then start documenting it :)

[adelmemariani]

done!