Add consumer-based RfC Visibility settings

This setting will be useful to increase data protection, where users might not be allowed to see RfCs from other contexts.

app/models/consumer.rb

@@ -1,6 +1,12 @@
 # frozen_string_literal: true
 
 class Consumer < ApplicationRecord
+  enum rfc_visibility: {
+    all: 0,
+    consumer: 1,
+    study_group: 2,
+  }, _default: :all, _prefix: true
+
   has_many :users
   has_many :study_groups, dependent: :destroy