Implement user system

Currently, if a client wants to use our application, they will need to get their own `client_id` and `client_secret`. We could generate dummy credentials and host it by keeping the `config` file hidden, that way nobody can get access to them.

The downside: If multiple clients use the application at once, it could run out of requests for that hour. Unlikely, but possible.

A more permanent solution would be to use `Satellizer` to have web tokens generated by the OAuth 2.0 procedure as Github requires, which will allow the user to authenticate with their accounts and make requests in their name. Also, this paves the way for user systems and the subscription manager. I would like to make this a priority.

The issue names no files, tests, or entry points. Start by clarifying the intended Satellizer and GitHub OAuth 2.0 flow, credential handling, and user-system scope; done requires an agreed implementation boundary and working user authentication.

Written by the indexing model from the issue text.