Allow signup and login from GitHub

[nikochiko]

Tasks:

• Should allow user to signup/login quickly from GitHub without requiring any additional input.

Implementation:

• The main backend access point for frontend would be the loginWithGithub Mutation. It's schema would be like

  mutation loginWithGithub(code: String!) {
  # Similar to normal `register` mutation
  success: Boolean!
  errors: [String!]
  # login: LoginMutation  -- edited
  token: String
  user: UserType
  }

• A new GithubAuthUser table will be defined with only a foreign key to the main Users table, and it's primary key as the github's user id.
• The backend will use the oauth authorization code to get the github token
• It will try to get the user's details with that token (the name, github's user id, and emails).
• The github id will be matched against the table for GithubAuthUser. If the user exists, it will return that user.
• If not, we will get the primary verified email of the user and validate that (mostly a check for uniqueness). I haven't exactly planned the frontend flow for when a user with that email already exists, but I think displaying an error with details would be appropriate.
• If the email is available, a new user should be created with new_user=True and login_type='github', and a new GithubAuthUser will be created.
• The new User will have a random readable username (with some username generator), a random password, and the user's primary verified email as email.
• The new_user flag should be made False and login_type to both or github password when the user changes their username.

New TODOs:

• [x] Add a postgres JSONField for login_types and make the login_types visible in a given format with a resolver
• [x] Add a mutation for changing username of user which will toggle new_user to False.
• [ ] Add "password": true to login_types when a password is set on the User. For this, override the User's default set_password method to send a password_changed signal.
• [x] Add mutation to associate already existing user with a GitHub account.