Refactor settings model

Move the various sections to their own concerns, so they're easier to
find and maintain

app/models/concerns/settings/open_collective_settings.rb

@@ -0,0 +1,9 @@
+module Settings
+  module OpenCollectiveSettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :opencollective_enabled, type: :boolean, default: true
+    end
+  end
+end