make backend domain name/location configurable

[MoralCode]

currently the frontend is hardcoded to use the api.classclock.app domain name for fetching schedule data.

this is a problem for people that want to host the app themselves on their own domain.

Implementation ideas: possibly use an env var or superadmin setting (future) or something

• [ ] Can ClassClock run without a domain? if so what should the default configuration be (maybe assume the API is on a specific port at the same location as the frontend)?
• [ ] What are the CORS rules? does the backend need to be on the same domain? The backend probably needs this too in order to give the right CORS exception data