Allow turning keybase off instance-wide

app/controllers/well_known/keybase_proof_config_controller.rb

@@ -2,8 +2,16 @@
 
 module WellKnown
   class KeybaseProofConfigController < ActionController::Base
+    before_action :check_enabled
+
     def show
       render json: {}, serializer: ProofProvider::Keybase::ConfigSerializer
     end
+
+    private
+
+    def check_enabled
+      head 404 unless Setting.enable_keybase
+    end
   end
 end