Attach serenity client to SpacebarBackend

[bitfl0wer]

Once registration and login (#8) have been implemented in the rust backend, we can use the token we get from these functions to create a serenity client, which should do all the heavy lifting of connecting to a websocket etc. for us. The serenity client should be added to the backend::SpacebarBackend.

[bitfl0wer]

Issue more complex than I thought at first. I assumed that serenity.rs would provide proxies for the Gateway and CDN as well, but it seems like only the API supports being proxied out of the box. The CDN and Gateway URLs are hardcoded within serenity. I will have to find a good way to fix that. :/ Could take a while. If this doesn't work: We either have to use a different library or code a new library from the ground up. Yay

[bitfl0wer]

Client has been attached in latest commit of the issues' branch. Functionality needs testing though, especially since I am a dum-dum who hasn't tested the changes made to the serenity repo :3