Define methods directly for "GraphQL Type authorization"

The following discussion from !25724 (merged) should be addressed:

• @godfat started a discussion:

  I just realized that we don't need to build a lambda for build_checker at all? We can just call it as a method.

  verify(ctx[:current_user], resolved_obj)

  And define it like:

  def verify(current_user, object)
    # everything in the previous build_checker
  end

  We don't need that check lambda either:

  def check(current_user, object)
    authorizations.all? do |ability|
      Ability.allowed?(current_user, ability, object)
    end
  end

  I wrote those as lambdas because I was trying to follow the old code, but it looks like we really don't need that. Is it right?

  Anyway, I think we can do this later. This is already well done and not exactly related to this merge request.