Added an improved CORS implementation #1265

[Eric-Wasson]

After this has been reviewed and before releasing I would suggest to do two steps:

• Include a hint in the release notes to give a heads up about the new .env variable, so users can add their frontend there after upgrading and don't run into CORS errors.
• Change the default warning in the frontend ("Network error. Please verify the IP address (localhost:8080) and try again. Note: APIv2 HASHTOPOLIS_APIV2_ENABLE=1 needs to be enabled. ") to include a hint about the CORS setting (e.g. "Please make sure to include the used frontend in the .env file.").

Note: because multiple values in the Allow-Origin header aren't allowed, a small workaround for getting the currently used frontend has been used (1 2)