feat: Improved callbacks

[rhatherall]

Callbacks are now named better and provide access to the request instead
of the session. Requests are more useful than sessions and still provide
access to the session should you need it.

The on_valid_login callback has been renamed to after_login and the
session is replaced with a request object. For example:

config.after_login = lambda do |token, user_info, request|
  user = User.where(identifier: user_info.sub).first_or_create do |user|
    user.email = user_info.email
  end
  request.session[:user_id] = user.id
end

The on_logout callback has been renamed to before_logout and the
session is replaced with a request object. For example:

config.before_logout = lambda do |request|
  # Your last chance to do something before the session is reset.
end

BREAKING CHANGE: The callback signatures have changed. Initializers must
be updated.