Handle multiple requests in parallel

[frosch123]

Currently the frontend can only handle one request at a time, while waiting for the API.

Two solutions:

• Make the front-end really state-less, so multiple instances can run in parallel. That is, store all session information in cookies. However, don't use Flask's default sessions. While they sign the data to prevent modification, they still expose somewhat-secrets like the API tokens. Encrypt the data instead.
• Reimplement the frontend with aiohttp.