whitelists and blacklists can be global

Until now, Feature.whitelist and .blacklist took a required first
argument -- the name of the feature:

    Arturo::Feature.whitelist(:foo) do |recipient|
      recipient.plan.has_foo?
    end

Now whitelists and blacklists can be global. The block takes the feature
as the first argument:

    Arturo::Feature.whitelist do |feature, recipient|
      recipient.plan.has?(feature.to_sym)
    end
FeaturesController: fix docs

The docs erroneously said to override `#permitted?`. The correct method
name is `may_manage_features?`.