move data handler to a web worker

Referring to [data_handler.js](https://github.com/uva-bi-sdad/community/blob/main/dev/data_handler.js). This is set up to be a web worked at least in that it's copied to each site, but might require some extensive reworking, based on the way it interacts with the core code ([community.js](https://github.com/uva-bi-sdad/community/blob/main/dev/community.js)).