Add chat server selector

[banool]

Unless we go with some fancy different chat backend where there's a shared chat server via relays or something, it is likely that people might want to run chat servers of their own. For this there should be a backend server selector where you can input the backend URL.

It should be smart enough to add the necessary http prefixes / endpoints / etc. based on whatever the user inputs. As in, if they forget to add /chat for example, we should add it for them.