Add ability to log in to discord on frontend

[cfi2017]

The discord auth token will be sent in a request to the backend in order to authenticate the user with the backend.

After logging in, the authentication information will be sent to the backend with every request, where it will be verified, if necessary by sending a request to the discord api.

This way, it can be guaranteed that a user is indeed who they say they are.

[cfi2017]

This was adressed in 8617343, it is however still WIP.